In English they are known as 'conditionals', and we've got a few of them. Each one uses a different set of tenses and conditional words that express a different meaning. Essentially this is how we express what we 'would', 'will' or 'would have' done 'if' something else had taken place. Hence, these are 'conditional' statements. For the foreign learner of English, these are often among the trickiest of grammatical turns of phrase; they require a grasp of English tenses (of which there are many) and a definite level of speaking confidence in order to practice.
In Polish they aren't actually quite as difficult (which is unusual), because there aren't as many combinations of participle tenses to get to grips with. But this doesn't mean they are a breeze either (I mean come on, what is in Polish grammar?!). Let's take a look at the various rules of grammar, syntax and the required vocabulary to get you using speculative sentences that require the conditional in Polish.
Speculating on Unlikely Events, And Other Uses of the 'Simple Conditional' in Polish
One major use of the conditional is to speculate on unlikely events, and the example that's most often used in, perhaps both, life and the language classroom, is the sentence 'what would you do if you won the lottery?'. In Polish this is 'co byś zrobił, gdybyś wygrał na loterii?', where the word 'byś' signifies this is question is referring to a possible event in the future ('would', as opposed to 'would've'), while the word 'gdybyś', is the conditional 'if', in the first person.
To answer, the conditional needs to be formed from the verb that you intend to use to describe your projected actions. So, following the example, where one may decide to buy something extravagant, you could answer, 'kupiłbym sobie samochód' (I would buy myself a car). Notice here that, unlike in English, where the verb in the conditional remains separate from the conditional word itself (I would buy...), in Polish, the verb ending undergoes a conjugation.
The conjugation follows a simple rule, whereby the third person singular (or third person plural for plural subjects) past tense form is given a variety of personal endings to indicate the subject. Using the verb 'to buy' from the example, the singular endings are as follows: kupiłbym (I would buy), kupiłbyś (you would buy), and kupiłby (he/she would buy). Plural endings require separate conjugations for males and females (following the rules of past tense verb conjugation), the masculine endings (to indicate the required personal endings) are as follows: kupilibyśmy (we would buy), kupilibyście (you [pl] would buy), and kupiliby (they would buy).
You can use conditionals like this to express a whole range of things in Polish, from asking politely to expressing desire and prohibition. This all essentially hinges on getting used to using the conjugation that indicates the addition of the English 'would' to a Polish sentence. One common example is when asking for things in shops (literally saying, 'I would like...'): Chciałbym chleb, for example, means 'I would like some bread'.
That said, it's important - as with many grammar rules in Polish - that you don't consider the conditional conjugation entirely transferable; there are some instances, when 'would' is used in English, that simply won't translate to use of the conditional conjugation in Polish. This is mainly to do with the versatility of English tenses, where the ostensibly present tense 'would' of English can actually be applied to a past tense sentences. 'He said he would eat the fish', for example, is not a possibility in Polish language.