Bevor ich den Mobilfunktarif wechsle, prüfe ich den Tarifvergleich.

Because bevor introduces a subordinate clause. In German, the finite verb usually goes to the end of a subordinate clause.

• Bevor ich den Mobilfunktarif wechsle
• literally: Before I the mobile-phone-plan switch

That is normal German word order after conjunctions like bevor, weil, dass, wenn, and obwohl.

In a main clause, German normally puts the finite verb in second position.

Here, the whole subordinate clause comes first:

• Bevor ich den Mobilfunktarif wechsle, ...

That entire clause counts as position 1. So the verb of the main clause must come next, in position 2:

• prüfe ich den Tarifvergleich.

This is a very common German pattern:

• Wenn ich Zeit habe, gehe ich spazieren.
• Weil es regnet, bleibe ich zu Hause.

Because both nouns are masculine singular and are being used in the accusative case as direct objects.

• der Mobilfunktarif → den Mobilfunktarif
• der Tarifvergleich → den Tarifvergleich

The verbs wechseln and prüfen both take a direct object here:

• einen Tarif wechseln
• einen Vergleich prüfen

So the article changes from der to den in the accusative.

Mobilfunktarif is a compound noun:

• Mobilfunk = mobile communications / cellular service
• Tarif = tariff, rate, plan

So Mobilfunktarif means something like:

• mobile phone plan
• cell phone plan
• mobile tariff

German uses compound nouns very often, so long words like this are normal.

It is another compound noun:

• Tarif = plan / rate / tariff
• Vergleich = comparison

So Tarifvergleich means plan comparison or comparison of tariffs/plans.

In more natural English, depending on context, it could mean:

• a comparison of phone plans
• a tariff comparison
• a rate comparison tool

No, in standard German the subject normally has to be stated in each clause.

So you say:

• Bevor ich den Mobilfunktarif wechsle, prüfe ich den Tarifvergleich.

Even though both clauses have the same subject, German still repeats ich. English does this too in a sentence like Before I switch..., I check...

Yes. You can also say:

• Ich prüfe den Tarifvergleich, bevor ich den Mobilfunktarif wechsle.

This means the same thing. The main difference is emphasis and sentence flow.

Notice the word order:

• after bevor, the verb still goes to the end: ... bevor ich den Mobilfunktarif wechsle
• in the main clause, the finite verb stays in second position: Ich prüfe ...

Because German uses a comma to separate a subordinate clause from a main clause.

Since bevor ich den Mobilfunktarif wechsle is a subordinate clause, it must be separated by a comma from the main clause:

• Bevor ich den Mobilfunktarif wechsle, prüfe ich den Tarifvergleich.

This comma is required in standard German.

In this sentence, wechseln means to change/switch from one thing to another.

• den Tarif wechseln = to switch plans

By contrast, ändern usually means to alter/modify something rather than replace it entirely.

So:

• Ich ändere den Tarif could suggest making changes to the plan
• Ich wechsle den Tarif means choosing a different plan

For phone contracts, wechseln is the more natural verb.

Yes. That would often sound very natural:

• Bevor ich meinen Mobilfunktarif wechsle, prüfe ich den Tarifvergleich.

The difference is:

• den Mobilfunktarif = the mobile plan
• meinen Mobilfunktarif = my mobile plan

Using meinen makes it more personal and often more idiomatic if you are talking about your own contract.

prüfen means to check, to examine, or to review. It sounds a bit careful or deliberate.

So this sentence suggests:

• first I carefully check the comparison,
• then I switch the plan.

Depending on context, German might also use:

• ansehen = to look at
• vergleichen = to compare
• überprüfen = to check/verify more thoroughly

But prüfen is perfectly correct and natural.

Yes. Bevor means before, and it introduces something that happens later than the action in the main clause.

In this sentence:

• prüfe ich den Tarifvergleich = I check the tariff comparison
• bevor ich den Mobilfunktarif wechsle = before I switch the mobile plan

So the checking happens first, and the switching happens afterward.

A very literal translation would be:

• Before I the mobile-phone-plan switch, check I the tariff comparison.

That sounds wrong in English, but it helps show the German structure:

1. bevor starts a subordinate clause
2. the verb wechsle goes to the end of that clause
3. the whole subordinate clause comes first
4. the main verb prüfe then takes second position in the main clause

So the sentence is a good example of two important German word-order rules happening at once.