Learn German: Das Auto fährt langsam, aber die U-Bahn ist schnell. - The car is driving slowly, but the subway is fast.

Fährt comes from the verb fahren (to drive or go). Fahren is a strong verb, which means its main vowel changes in the present tense for the du (you) and er/sie/es (he/she/it) forms. The a changes to an ä when we talk about a third-party thing like das Auto. For ich or wir, it keeps the regular a (like ich fahre).

The translation says "is driving", but isn't fährt just "drives"?

Yes, it is! German doesn't have a separate "-ing" tense (continuous tense) like English does. Das Auto fährt can mean both "The car drives" and "The car is driving" depending on what makes the most sense in English.

Why doesn't the word aber change the word order in the second half of the sentence?

Aber (but) is a connecting word that simply links two normal sentences together without messing up the word order. In German grammar, words like aber, und (and), and oder (or) take up "position zero". This means we start counting positions after them: die U-Bahn is position 1, and the verb ist stays safely in its normal spot at position 2.

What does the "U" in U-Bahn stand for?