Często mrużę oczy.

Because Polish often leaves out subject pronouns when the verb already shows who is doing the action.

Mrużę means I squint / I narrow, and the ending -ę tells you it is 1st person singular: I.

So:

• Często mrużę oczy. = I often squint my eyes / I often squint.
• Ja często mrużę oczy. = also correct, but ja adds emphasis, like I in I often squint my eyes.

In neutral Polish, leaving out ja is more natural.

Mrużę is the 1st person singular present tense form of the verb mrużyć.

So:

• mrużyć = to squint / to narrow / to half-close
• mrużę = I squint / I am squinting

This verb is imperfective, so it works well for:

• habitual actions
• repeated actions
• ongoing actions in context

That fits well with często (often).

In Polish, mrużyć oczy is a very common and natural expression. Literally it is to squint/narrow the eyes.

English often uses just to squint, but Polish commonly keeps the object:

• mrużyć oczy = to squint / to narrow one’s eyes

So Często mrużę oczy is very natural Polish.

Because with body parts, Polish often does not use a possessive word like my, your, his, etc., if it is already obvious whose body part is meant.

So Polish prefers:

• mrużę oczy = literally I squint eyes

But the natural meaning is:

• I squint my eyes
• or simply I squint

If you said mrużę moje oczy, it would usually sound unnecessary unless you were emphasizing my eyes in contrast to someone else’s.

Here oczy is in the accusative plural, because it is the direct object of the verb mrużyć.

You are doing the action to the eyes, so Polish uses the direct object case.

A useful pattern is:

• infinitive: mrużyć
• object: oczy
• full phrase: mrużyć oczy

Also, oko (eye) has an irregular plural:

• singular: oko
• plural: oczy

In this sentence, the accusative plural looks the same as the nominative plural: oczy.

Because oczy and oczu are different case forms.

For oko:

• oczy = nominative/accusative plural
• oczu = genitive plural

In Często mrużę oczy, the verb needs a direct object, so the accusative plural is required:

• mrużę oczy = correct

You would use oczu in different structures, for example after certain prepositions or in genitive contexts.

Yes, Polish word order is flexible, but the most neutral version here is:

• Często mrużę oczy.

This puts często (often) before the verb, which sounds very natural.

Other orders are possible, but they may sound more marked or emphasize something different:

• Mrużę oczy często. = possible, but more emphasis on often
• Oczy często mrużę. = possible, but strongly marked/emphatic

So for a learner, Często mrużę oczy is the safest and most natural order.

Często means often / frequently. It tells you that the action happens regularly or many times.

So the sentence describes a habit or repeated behavior:

• Często mrużę oczy. = I often squint my eyes / I often squint.

Because of często, this is not mainly about one single moment right now. It is more about what happens repeatedly.

A rough pronunciation guide:

• często ≈ CHEN-sto
• mrużę ≈ MRU-zheh
• oczy ≈ O-chih

A few important sounds:

• cz sounds like ch in chop, but harder and more retroflex
• ż sounds like the s in measure
• rz usually sounds the same as ż
• ę is a nasal vowel; at the end of a word, it is often pronounced only lightly nasally in everyday speech

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

• CZĘ-sto
• MRU-żę
• O-czy

Yes. Mrużyć is imperfective, which is why it works naturally with często.

A common related perfective verb is:

• zmrużyć oczy = to squint / narrow the eyes once
• sometimes also przymrużyć oczy = to half-close / squint the eyes

Very roughly:

• mrużyć oczy = repeated, ongoing, habitual
• zmrużyć oczy = one completed action

That is why Często mrużę oczy uses mrużę, not a perfective form: often usually goes with repeated or habitual actions.