Drenaż to ważna kwestia.

In this sentence, to works like a linking word: it connects drenaż and ważna kwestia.

So Drenaż to ważna kwestia is a very common Polish pattern meaning roughly:

• X is Y
• As for X, it is Y
• X is an important issue

It is not exactly the same word as jest, but in many cases it plays a similar role in this kind of sentence.

A useful pattern is:

• To jest dom. = This is a house.
• Polska to kraj w Europie. = Poland is a country in Europe.
• Drenaż to ważna kwestia.

Polish often uses to in definitions, identifications, and general statements.

Because ważna agrees with kwestia, and kwestia is a feminine noun.

In Polish, adjectives must match the noun they describe in:

• gender
• number
• case

Here:

• kwestia = feminine singular
• so ważny changes to ważna

Compare:

• ważny problem = an important problem
• ważna kwestia = an important issue
• ważne zagadnienie = an important topic/question

So the adjective is not matching drenaż here; it is matching kwestia.

In the X to Y structure, both nouns are usually in the nominative (dictionary) form.

So here you get:

• drenaż — nominative
• kwestia — nominative

That is normal for this construction.

This is one reason why to sentences are often easier for learners than sentences with jest, because with jest, the second noun phrase is often put in the instrumental:

• Drenaż to ważna kwestia.
• Drenaż jest ważną kwestią.

Both are correct, but the grammar is different.

Yes. That is also correct and very natural.

The difference is mainly grammatical and a little stylistic:

• Drenaż to ważna kwestia.
  A very common, straightforward X is Y structure.

• Drenaż jest ważną kwestią.
  Also means Drainage is an important issue, but now the predicate noun phrase is in the instrumental:
  ważną kwestią

For a learner, it helps to remember:

• X to Y → usually nominative after to
• X jest Y → often instrumental for the noun phrase after jest

Drenaż is masculine singular.

You can see that from its form and from how it behaves in sentences. For example:

• Ten drenaż jest potrzebny. = This drainage is necessary.
• Drenaż był skuteczny. = The drainage was effective.

Even though drenaż is masculine, that does not affect ważna in your sentence, because ważna belongs with kwestia, not with drenaż.

Polish word order is fairly flexible, but not every version sounds equally natural.

The most neutral version is:

• Drenaż to ważna kwestia.

You could also say:

• To ważna kwestia. = It’s an important issue.
• Ważna kwestia to drenaż. = An important issue is drainage.
  This is grammatical, but it changes the emphasis.

So yes, word order can change, but the original sentence is the most natural if you want to present drenaż as the topic.

A rough pronunciation is:

• drenaż ≈ DREH-nazh

A few helpful notes:

• dr is pronounced clearly, as in a consonant cluster
• e is like e in met
• naż ends with ż, which sounds like the s in measure

Stress in Polish is usually on the second-to-last syllable, so here the stress is on dre:

• DRE-naż

Yes, kwestia is a normal and natural word. It often means:

• issue
• matter
• question (in the sense of a topic, not a sentence ending with a question mark)

So ważna kwestia is a very natural phrase meaning an important issue.

Some similar words are:

• sprawa = matter, issue, affair
• problem = problem
• zagadnienie = issue/topic, often a bit more formal or technical

In this sentence, kwestia sounds perfectly normal.

Because Polish does not have articles like English a/an and the.

So ważna kwestia can mean:

• an important issue
• the important issue

Which one is meant depends on context.

That is completely normal in Polish, and learners just have to get used to understanding definiteness from the situation rather than from an article.

It is mostly neutral.

• drenaż is a somewhat technical word, depending on context
• to ważna kwestia is a very standard, natural way to make a statement

So the whole sentence would fit well in:

• everyday speech
• practical discussion
• written explanation
• technical or semi-formal contexts

It does not sound especially slangy or especially literary.