For tre år siden gik jeg sjældent i teatret, men nu går jeg flere gange om året.

For tre år siden is a fixed expression meaning three years ago.

• for tre år siden = a point in time, counted back from now
• i tre år = for three years, meaning duration

So:

• For tre år siden gik jeg sjældent i teatret = Three years ago, I rarely went to the theatre
• Jeg gik i teatret i tre år = I went to the theatre for three years

That difference is very important in Danish.

Because Danish main clauses usually follow the V2 rule: the finite verb comes in second position.

Here, For tre år siden is placed first in the sentence, so the verb must come next:

• For tre år siden + gik + jeg ...

If the subject came first, you would get:

• Jeg gik sjældent i teatret for tre år siden

Both are possible, but the sentence you were given starts with the time expression, so inversion happens.

They are different because the sentence contrasts past habit and present habit.

• gik = past tense of gå
• går = present tense of gå

So the structure is:

• For tre år siden gik jeg ... = in the past
• men nu går jeg ... = now / at present

English often does the same kind of contrast:

• I rarely went
• but now I go

Yes. At gå is irregular, so its forms do not follow the simplest pattern.

Common forms are:

• infinitive: at gå
• present: går
• past: gik
• past participle: gået

So gik is just something you need to learn as part of the verb.

It means to go to the theatre or to attend theatre performances.

This is an idiomatic Danish expression. Danish often uses gå in expressions about regularly going somewhere as an activity, not just physically walking.

So here gå does not mean walk. It means go in the sense of attending.

For the activity going to the theatre, Danish normally says gå i teatret.

• i teatret = attending the theatre as an activity
• til teatret = more like movement toward the theatre building, and it is not the normal expression here

So in this sentence, i teatret is the natural choice.

A useful comparison:

• gå i biografen
• gå i teatret

These are standard activity expressions.

Because teater is a neuter noun.

Its singular forms are:

• et teater = a theatre
• teatret = the theatre

In the expression gå i teatret, Danish uses the definite form. This does not necessarily mean one specific theatre building. It is just the normal idiomatic form in this expression.

Sjældent means rarely, and it normally comes after the finite verb and subject, before the rest of the phrase.

Examples:

• Jeg går sjældent i teatret
• For tre år siden gik jeg sjældent i teatret

So in your sentence, sjældent is in a very normal position.

Here nu means now / nowadays / at present.

It does not have to mean this exact moment. In this sentence, it contrasts the speaker's life before with the speaker's life now:

• For tre år siden ...
• men nu ...

So the meaning is closer to these days than to at this exact second.

It means several times a year.

Breakdown:

• flere gange = several times
• om året = per year / a year

Here flere often corresponds to English several, not just literal more.

So:

• Jeg går flere gange om året i teatret = I go to the theatre several times a year
• Jeg går i teatret flere gange om året = same meaning

Om is used in Danish to express frequency over a repeated period, like per in English.

Examples:

• om dagen = per day
• om ugen = per week
• om måneden = per month
• om året = per year

So flere gange om året literally means something like several times per year.

Because Danish still follows the V2 rule in the second main clause.

After men, the clause itself begins with nu, so nu takes first position and the verb must come second:

• men nu går jeg ...

Not:

• men nu jeg går ...

Also, men does not count as the first element inside the clause for V2 word order. It simply connects the two clauses.

Yes, absolutely. That is also natural Danish.

Compare:

• Nu går jeg flere gange om året i teatret
• Nu går jeg i teatret flere gange om året

Both are understandable and natural. The difference is mainly one of rhythm and emphasis. The version in your sentence puts the frequency phrase earlier and leaves i teatret closer to the first clause, which helps the contrast read smoothly.

Because men joins two main clauses, and in standard Danish writing you normally put a comma before it.

So:

• For tre år siden gik jeg sjældent i teatret, men nu går jeg flere gange om året.

That comma is standard punctuation.