Wczoraj kupiłam brązową torbę, bo jest praktyczniejsza niż ta szara.

Because the speaker is female.

In the past tense, Polish verbs agree with the gender of the subject:

• kupiłam = I bought (said by a woman)
• kupiłem = I bought (said by a man)

So this sentence tells you that the speaker is a woman.

Because torbę is the direct object of kupiłam.

After a verb like kupić (to buy), the direct object usually goes into the accusative case. For a feminine singular noun, that changes both the noun and the adjective:

• brązowa torba = nominative, the basic dictionary form
• brązową torbę = accusative, used here after kupiłam

So:

• torba → torbę
• brązowa → brązową

Polish often leaves out subject pronouns when they are clear from context.

In English, you usually need it is. In Polish, you can simply say jest if the listener already knows what you mean.

Here, the understood subject is the brown bag:

• bo jest praktyczniejsza... = because it is more practical...

The it is not stated, but it is understood.

Because Wczoraj applies to the buying, not necessarily to the comparison.

The sentence means that yesterday the speaker bought the bag, and the reason for that purchase is that the bag is more practical. The bag is still more practical now, so the present tense makes sense.

In other words:

• Wczoraj kupiłam... = the buying happened yesterday
• bo jest praktyczniejsza... = the bag is more practical in general / now

If you wanted to focus on a past situation only, była could be possible in a different context, but jest is very natural here.

It is the comparative form of praktyczna (practical, feminine).

Polish often forms comparatives by adding endings like -szy / -iejszy / -niejszy, depending on the word. Here:

• praktyczna = practical
• praktyczniejsza = more practical

Because it describes a feminine noun (torba), it uses the feminine form:

• masculine: praktyczniejszy
• feminine: praktyczniejsza
• neuter: praktyczniejsze

Yes.

Both are possible:

• praktyczniejsza = more practical
• bardziej praktyczna = more practical

The synthetic comparative praktyczniejsza is very natural here and often sounds a bit more compact. bardziej praktyczna is also correct and common.

Niż means than in comparisons.

Here:

• praktyczniejsza niż ta szara = more practical than the grey one

After niż, Polish very often uses the nominative, especially in standard modern Polish. That is why you get:

• ta szara

not accusative tę szarą in this sentence.

Because this phrase stands after niż and is treated as a comparison in the nominative.

Also, the noun torba is omitted because it is already understood. So:

• ta szara really means ta szara torba = that grey bag

The forms are nominative feminine singular:

• ta = that
• szara = grey

So ta szara means the grey one / that grey one.

Yes, very often.

If the noun is obvious from context, Polish can leave it out and keep only the demonstrative and adjective. English does something similar with the grey one.

So:

• ta szara = that grey one
• understood noun: torba

This is completely natural.

Because bo introduces a reason clause, like because in English.

Polish normally puts a comma before conjunctions such as:

• bo = because
• że = that
• ale = but

So:

• Wczoraj kupiłam brązową torbę, bo jest praktyczniejsza niż ta szara.

That comma is standard punctuation.

Yes, Polish word order is flexible.

Starting with Wczoraj puts time first and sounds very natural:

• Wczoraj kupiłam brązową torbę...

But other orders are also possible, for example:

• Kupiłam wczoraj brązową torbę...

The difference is mostly about emphasis, not basic meaning. Polish uses word order more freely than English.

Because kupiłam is from the perfective verb kupić, which presents the action as completed.

• kupiłam = I bought / I did buy / I ended up buying
• kupowałam = I was buying / I used to buy / I was in the process of buying

Here the speaker means a completed single action yesterday, so kupiłam is the right choice.

Because they all refer to torba, which is a feminine noun.

In Polish, adjectives agree with the noun in gender, number, and case.

So:

• torba is feminine singular
• adjectives referring to it must also be feminine singular

But the exact ending changes depending on the case:

• brązowa torba = nominative
• brązową torbę = accusative

And in the comparison:

• praktyczniejsza refers to the brown bag
• ta szara refers to the grey bag

Both are feminine because torba is feminine.