Well, here I am again.
Powinien is a very difficult czasownik for learners of Polish at the best of times, but when it comes with a conditional ending as well.
I was doing some exercises where you had to replace the verb in the first sentence with either ; trzeba, można or warto. So far so good , but then I saw this example and immediately considered suicide.

Question ; Powinni byśmy zaprosić Kowalskich . Answer ; Trzeba by zaprosić Kowalskich.

Firstly, what does powinni byśmy mean and secondly, what is ' by ' doing with trzeba? I've never seen that before

I believe this "Powinni byśmy' would be something like "we should" (it means exactly the same as "powinniśmy", which is a shorter form and to be honest, I've never heard anyone say "Powinni byśmy", although Im a native speaker of Polish). Id rather say either "Trzeba by" (like in your answer) or simply "Powinniśmy".

As for the "by" with "trzeba", I believe the "by" puts "trzeba" in a tryb przypuszcający?
If the sentence was "Trzeba zaprosić Kowalskich", it would mean "We HAVE TO invite Kowalscy". "Trzeba by" is more like "We SHOULD invite Kowalscy".

Hope this helps!

I agree with gremlinekdance. The starting sentence sounds very artificial in the present
tense, while "Trzeba by zaprosić Kowalskich" sounds natural. You could also say
"Należałoby zaprosić Kowalskich " or "Wypadałoby zaprosić Kowalskich" (this time
"by" is written jointly with the preceding word). However, a similar sentence in
the past tense would be an entirely different story "Powinni bylibyśmy zaprosić
Kowalskich", which closely resembles the English "We should have invited
the Kowalskis".

Firstly, many thanks for two brilliant answers which help a lot. Can I just ask about the example Muffin 666 gives in the past tense ? What's the difference between: powinni bylibyśmy zaprosić Kowalskich, which is a conditional past tense and powinni byliśmy zaprosić Kowalskich, which is a normal past tense? I'm still a bit confused , because I thought powinien had a sort of conditional meaning anyway, English should or ought, so why the need to give it a conditional ending in the past tense?

I am not a linguist and I cannot give you a precise technical explanation, just my own feelings.
"Powinni bylibyśmy" may sound slightly old fashioned, but emphasizes well the past nature of the missed
opportunity. I believe this is a remnant of something similar to past perfect, that does not exist in the Polish
language any more. I think both forms are acceptable, but "powinni bylibyśmy" sounds better to me.
Be aware though that I tend to be linguistically conservative.

Firstly , may I say that I think you are incredibly modest to say you're not a linguist. I assume you're a native Polish speaker and take my hat off to your very fluent and excellent English. I am aware that the past perfect ,or pluperfect as I think it's also called , is not really used in Polish anymore , but I'm just really interested in this aspect of Polish grammar. We had a long thread on this very subject a couple of years ago on this forum. What about powinni byliśmy byli Can you tell me please if this makes any sense?

Thanks a lot for your kind words. I am not a linguist indeed, but I love literature poetry, all aspect of language evolution, communication and last but not least etymology. I am an electrical engineer and a software developer raised and educated in Poland. I have been living in Canada for more than a couple of decades. I am still professionally active, but far from being a youngster any more. I am writing all that so you, and others, can qualify my opinions better. The last think I desire is to mislead people.

As far as your question is concerned, I think "powinni byliśmy byli" is OK, but it overemphasises the past nature of the expression as both byliśmy and byli indicate status conditions in the past. To me, it resembles an expression like
"Poszedłem był do cukierni na Nowym Świecie" that would be perfectly acceptable in a 19th century novel, but would raise some brows among young contemporary Poles. Hearing an expression like quoted by me (not necessarily yours) would be probably viewed as snobbish by some.

Thank you for sharing your very interesting background with us. You're clearly a very intelligent individual and I'm very grateful to you for taking the time to give me such interesting and informative answers to my questions. I should add that I'm far from being a youngster myself, but have developed a real interest in Poland and it's culture( especially Polish beer!) since the recent large influx of Poles into the UK.

Ok, so I am technically a linguist (although not in Polish), but I don't have a whole lot to do with the profession, in fact I don't deal with humanities (science is sooooo much better he he) in general anymore. But let me add my two cents, or at least share how it looks from my perspective.

"Powinni byliśmy byli" is definitely not right. And the reason for that would be doubling of "byli". It'd be like saying: "I would have gone have" or "I would have gone went". Do you know what I mean?

The structures you're talking about are very complicated and I wouldn't even attempt to explain them. But a lot of them are old-fashioned and not used anymore. Some structures mean the same thing, too.

So using the examples below:

"powinni bylibyśmy" - correct. Means that we should do something.

I'm not sure if "-by" determines "tryb przypuszczający" (Like in "trzebaby") though. It adds more of an obligatory aspect. Like we're socially or otherwise expected to do something. I'm not sure if I'm expressing myself well here. I know it's confusing. It kind of builds on what gremlinekdance said. I just wouldn't call it "tryb przypuszczający"

Also, I agree that "Powinni bylibyśmy zaprosić Kowalskich" sounds very artificial. It almost doesn't make sense to me. Are you sure it shouldn't be "Powinni byliśmy zaprosić Kowalskich"? Because that would be the equivalent to past perfect: "We should have invited Kowalskis". As I'm writing this, I keep repeating the first sentence to myself and I'm leaning more and more towards the opinion that it is just plain wrong. Maybe a misprint. Or some very old fashioned construct I'm not familiar with. If you're very adamant, I'll be in Poland in a few weeks, I can ask some "real" linguists.

Ok, sorry for rambling, this was kind of a stream of consciousness, so I hope it makes sense. If not, let me know.

OK. Quick engineering summary of how the phrases we have been discussing sound to me:
Powinni bylibyśmy – sounds OK to me, but slightly archaic
Powinniśmy byli – elegant, but not very contemporary
Powinni byliśmy byli – I like it. It sounds very archaic and might be considered pompous according to the current standards. Something in the nature of my favourite "poszedłem byłem wraz z panem hrabią do zamtuza"
Powinni byliśmy – sounds contemporary and likely to be used.

I'm truly fascinated by your comments and intend to run them by my Polish teacher here in England. I'll let you know what she says. Thanks indeed for your interest in my post

Hmmm...

It might be that you all have lost interest in this debate by now, but I have just stumbled across it and started wondering about something...

Given the, as it's been pointed out, highly archaic and grammatically complex nature of the discussed phrase, wouldn't it be more accurate to translate it to english as "we should have had ..." ? Just wondering and would be interested to know your answer. I am not linguist either, and although I've studied english for many years at school, after few years of living in UK I rather use it instinctively now and started questioning my grammar knowledge of the language
Many thanks.

Hi . I certainly haven't lost interest in this topic , I'm rather fascinated by it to be honest. Your translation of we should have had sounds exactly right to me. It's just so strange , as a native English speaker, to understand how a complete grammatical case, ie czas zaprzeszły, could have disappeared from the Polish language Thanks for your interest

Oh, I am fascinated by this forum now, can't get enough (sad...? ). I think the inner teacher in me has been awakened!

Well I guess that every language evolves with time and, sadly enough, they seem to have a tendency for simplification. Czas zaprzeszly forms pretty complicated phrases that weren't that easy to deal with, even for native Polish speakers. I guess that it sort of fell out of fashion eventually.

Hope to see you more often on here from now on then