Czasowniki modalne (modal verbs) are one of grammar issues recognized both in Polish and English. Generally speaking, they are used to express modalność (modality) and there are a few words and expressions that belong to this category.
Musieć (must) – in English it is used for logical assumptions when we are certain that something is true and as an obligation, duty e.g.
[In Polish]
On musi mieć/na pewno ma 16 lat.
(Ja) muszę odrobić zadanie domowe.
[In English]
He must be 16.
I must do my homework.
English has also other forms for necessity like have to and need, which is not the case in Polish where we use just the word musieć e.g.
[In Polish]
Dzieci muszą nosić mundurki w tej szkole.
Czy muszę do niego zadzwonić?
[In English]
Children have to wear uniforms in this school.
Need I call him?
To say that something happened in the past for sure we use must + perfect infinitive in English and the past form of the verb musieć or the expression na pewno in Polish e.g.
[In Polish]
Ona musiała to widzieć/na pewno to widziała.
[In English]
She must have seen it.
When we are certain that something is not true we use nie + the suitable form of the verb móc (can’t/couldn’t) or na pewno nie e.g.
[In Polish]
Oni nie mogą być/ na pewno nie są tutaj. (Oni) są we Włoszech.
[In English]
They can’t/couldn’t be here. They are in Italy.
And again, when we talk about the past event we use can’t/couldn’t + perfect infinitive in English and nie + the past form of the verb móc (can’t/couldn’t) or the expression na pewno nie e.g.
[In Polish]
Ona nie mogła oglądnąć/na pewno nie oglądała tego filmu.
[In English]
She can’t/couldn’t have watched this film.
Can is also used when we want to show someone’s umiejętność (ability) which in Polish is done by means of the verb móc/potrafić e.g.
[In Polish]
On umie/potrafi śpiewać.
(My) umiemy/potrafimy grać na gitarze.
[In English]
He can sing.
We can play the guitar.
When we are not certain about something and we do not know exactly whether something is or was true we have the verbs may, might and could whose Polish equivalent is the expression być może/może/możliwe, że + the verb in the future form e.g.
[In Polish]
Być może/Może/Możliwe, że on przyjdzie.
Być może/Może/Możliwe, że kupię nowy aparat.
[In English]
He May/might/could come.
I may/might/could buy a new camera.
To show this uncertainty in the past, we use may/might/could + perfect infinitive which is być może/może/możliwe, że + the past form of the verb e.g.
[In Polish]
Być może/Może/Możliwe, że wygrali ten konkurs.
[In English]
They may/might/could have won this competition.
This particular construction is also used to indicate that something could happen although it did not and then, in Polish we have the proper past form of the verb móc e.g.
[In Polish]
(Oni) mogli mieć wypadek. Jechali za szybko.
Mogłeś oblać ten egzamin. Nie uczyłeś się wcale.
[In English]
They may/might/could have had the accident. They were driving too fast.
You may/might/could have failed this exam. You did not learn at all.
In order to forbid something, the English language uses mustn’t/can’t which in Polish is expressed by nie można/nie wolno e.g.
[In Polish]
Nie można/Nie wolno tutaj parkować.
[In English]
You mustn’t/can’t park here.
The construction used for formal instructions is/are to in Polish is ma/mają + infinitive e.g.
[In Polish]
Nikt nie ma opuszczać pokladu.
Mają wypełnić te formularze.
[In English]
No one is to leave the board.
They are to fill in these forms.
There are also modal verbs like powinno się (should/ought to) which we use to give advice, but also to show what someone’s duty is e.g.
[In Polish]
Nie wyglądasz dobrze. Powinieneś iść do lekarza.
Prawnicy powinni znać dobrze prawo.
[In English]
You do not look well. You should/ought to go to the doctor.
Lawyers should/ought to know the law well.
Another expression used in this situation is lepiej, żeby + past form of the verb (had better + infinitive) e.g.
[In Polish]
Lepiej, żebyś odwiedziła swoją babcię. (Ona) jest w szpitalu.
[In English]
You’d better visit your grandma. She is in hospital.
To express krytyka (criticism) i.e. what somebody should do in the past, but he/she actually did not, there is the construction powinno się było (could/should/might/ought to + perfect infinitive) in the correct form e.g.
[In Polish]
Powinieneś był się więcej uczyć.
Powinnam była poprosić ją o pomoc.
[In English]
You could/should/might/ought to have learned more. I could/should/might/ought to have asked her for help.
Modality in Polish is not so expressive as it is in English. Nevertheless, it is a part of the language and without knowledge of some of the rules, the proper use of Polish would certainly not be possible.