Czasem mylę to wyrażenie z innym, które brzmi prawie tak samo.

Yes. Czasem means sometimes in this sentence.

You can also say czasami. Both are very common and natural:

• Czasem mylę...
• Czasami mylę...

There is little difference in meaning here; czasem is just a bit shorter and slightly more compact in style.

Because mylić and mylić się are related but not the same.

• mylić coś z czymś = to confuse one thing with another
• mylić się = to be mistaken / to be wrong

So in this sentence, the speaker is not saying I am mistaken in general. They are saying they confuse one expression with another expression, so the non-reflexive verb is needed:

• Mylę to wyrażenie z innym. = I confuse this expression with another.
• Mylę się. = I am mistaken.

Mylę is the 1st person singular present tense form of the imperfective verb mylić.

So:

• ja mylę = I confuse / I mix up

Because of czasem, the present tense here has a habitual meaning:

• Czasem mylę... = I sometimes confuse...

It does not mean only what is happening right this second. In Polish, present tense of an imperfective verb often describes repeated or usual actions.

Because the sentence describes something that happens from time to time, not one finished event.

• mylić = imperfective, good for habits, repeated actions, general statements
• pomylić = perfective, good for one completed mistake

So:

• Czasem mylę... = I sometimes confuse...
• Pomyliłem... = I confused... / I made a mistake and mixed up...

With czasem, the imperfective mylić is the natural choice.

This is a very useful pattern:

mylić coś z czymś = to confuse something with something

The cases are:

• the first thing: accusative
• the second thing after z: instrumental

In your sentence:

• to wyrażenie = the thing being confused → accusative
• z innym = with another one → instrumental

So the structure is:

• mylę [co?] to wyrażenie [z czym?] z innym

A full version would be:

• Mylę to wyrażenie z innym wyrażeniem.

Because wyrażenie is a neuter noun.

In Polish demonstratives agree with gender:

• ten for masculine
• ta for feminine
• to for neuter

Since wyrażenie is neuter, you say:

• to wyrażenie = this expression

Also, for this noun, the accusative looks the same as the nominative, so to wyrażenie keeps the same form here.

Innym is standing in for a fuller phrase:

• z innym wyrażeniem

Polish often omits a repeated noun when it is obvious from context. So instead of repeating wyrażeniem, the sentence just says z innym = with another one.

Why innym?

• because it goes with the omitted noun wyrażeniem
• and after z in this pattern, you need the instrumental
• inne wyrażenie → innym wyrażeniem

So innym is basically a shortened version of innym wyrażeniem.

Które is a relative pronoun meaning which / that.

Here it refers to the omitted noun inside innym:

• z innym [wyrażeniem], które brzmi prawie tak samo

So the idea is:

• with another expression, which sounds almost the same

It is które because wyrażenie is:

• singular
• neuter

So the relative pronoun must match that gender and number.

Because the case of the relative pronoun depends on its role inside the relative clause, not on the case of the noun before it.

In:

• które brzmi prawie tak samo

które is the subject of brzmi, so it is in the nominative.

Compare the hidden full structure:

• z innym wyrażeniem → instrumental
• które brzmi... → nominative, because which is doing the action of sounding

So even though innym is instrumental, które is nominative.

Literally, brzmi means sounds, from brzmieć = to sound.

So:

• brzmi prawie tak samo = sounds almost the same

A word-by-word breakdown:

• brzmi = sounds
• prawie = almost
• tak samo = the same / in the same way

This phrase is about sound, not spelling. It means the two expressions are very similar in how they sound when spoken.

Yes. In standard Polish, a subordinate clause introduced by który / która / które is normally separated with a comma.

So:

• ...z innym, które brzmi prawie tak samo.

That comma is the normal and correct punctuation.

Yes. Polish word order is fairly flexible, although some versions sound more neutral than others.

The original:

• Czasem mylę to wyrażenie z innym, które brzmi prawie tak samo.

This is natural and neutral.

You could also say:

• To wyrażenie czasem mylę z innym, które brzmi prawie tak samo.

That puts more focus on to wyrażenie.

So the meaning stays basically the same, but the emphasis can shift depending on word order.