В этой булочной ватрушка мягче, чем бублик, который я купила вчера.

Because в means in here, and when Russian uses в for location, it normally takes the prepositional case.

So both words change to prepositional singular feminine:

• эта → этой
• булочная → булочной

So:

• в этой булочной = in this bakery

A useful extra point: булочная is a noun, but it declines like an adjective, which is why its endings may look adjective-like.

In the present tense, Russian usually leaves out the verb to be.

So where English says:

• The vatrushka is softer than the bagel

Russian simply says:

• Ватрушка мягче, чем бублик

This is completely normal. A present-tense is/are/am is usually understood rather than spoken.

Мягче is the comparative form of мягкий.

• мягкий = soft
• мягче = softer

Russian often makes comparatives with a special word form instead of using a separate word like more.

So:

• мягкий = soft
• более мягкий = more soft / softer
• мягче = softer

In everyday Russian, мягче is the most natural choice here.

Because after чем in a straightforward comparison, Russian often keeps the second item in the nominative:

• ватрушка мягче, чем бублик

This literally works like softer than a bagel.

There is also another common pattern without чем, where the second noun goes into the genitive:

• ватрушка мягче бублика

Both are grammatical. The version with чем is very clear and often feels a bit more explicit to learners.

Because чем introduces the comparison, and in Russian this kind of comparative phrase is normally separated by a comma:

• мягче, чем бублик

Russian punctuation is often stricter than English punctuation in these structures, so the comma is expected here.

Который means which/that in a relative clause.

Here it refers to бублик, so the phrase means:

• the bagel which I bought yesterday

We know it refers to бублик because:

• бублик is masculine singular
• который is also masculine singular

It does not refer to булочная or ватрушка, because those are feminine. If it referred to one of those, you would expect которая instead.

This is a very common question.

Inside the relative clause, который is the direct object of купила:

• I bought which

So logically, it is in the accusative case.

But for a masculine singular inanimate noun, the accusative looks exactly like the nominative. That is why you see:

• который

not

• которого

Compare:

• бублик, который я купила = the bagel that I bought
• человек, которого я увидела = the person whom I saw

The second one uses которого because человек is animate.

Because Russian past-tense verbs agree with the gender of the subject.

So:

• я купил = I bought if the speaker is male
• я купила = I bought if the speaker is female

This sentence tells us the speaker is female.

Yes, it could move. Russian word order is flexible.

Here, вчера naturally goes with купила:

• который я купила вчера = which I bought yesterday

This is a very normal order. But Russian can move parts around for emphasis, for example:

• который вчера я купила
• вчера я купила бублик

The basic meaning stays the same, but the focus changes slightly. In your sentence, the end position for вчера sounds natural and neutral.

Russian often puts setting information first, such as place or time.

So starting with:

• В этой булочной

sets the scene first: in this bakery...

Then the sentence gives the main comparison:

• ватрушка мягче, чем бублик...

This word order is very natural. It highlights the location before making the statement.

Because который я купила вчера is a relative clause, and Russian normally separates relative clauses with commas.

So:

• бублик, который я купила вчера

This is the standard punctuation for noun + which/that... clauses in Russian.