Learn Hardcore German: Der Fahrer stoppt das Auto.

In German every noun is written with a capital letter, so Fahrer gets a capital F. The word Der is the definite article (the) in the masculine singular nominative case, marking Fahrer as the subject of the sentence.

Why is das Auto in the accusative case, and why is its article also das?

Auto is a neuter noun. In German the definite article for neuter singular is das in both nominative and accusative. Here das Auto is the direct object (accusative case), but the form of the article doesn’t change.

How is the verb stoppt formed from the infinitive stoppen?

Stoppen is a regular (weak) verb. For third-person singular (he/she/it) in the present tense you drop the ending -en and add -t:
stoppen → stoppt → stoppt.

Is stoppen a separable verb? If not, why doesn’t it split?

No, stoppen is a simple, inseparable weak verb. Only verbs with prefixes like an-, auf-, mit- etc. can be separable. Since stoppen has no separable prefix, it stays together in all tenses and moods.

How do you pronounce stoppt, especially the double p and final t?

You pronounce it like [ʃtɔpt]:

The st at the beginning is [ʃt] (similar to English “sht” in “shtick”).
The double p indicates a short “o” sound, [ɔ].
The final t is clearly released, unlike in English where word-final t’s can be softer.

Could I use hält (from anhalten) instead of stoppt, and what’s the difference?

Yes. Anhalten is a separable verb meaning “to stop” in the sense of halting movement. In the present you’d say hält … an, and you must split it:
Der Fahrer hält das Auto an.
Compared to stoppen, anhalten is often a bit more idiomatic for vehicles, but both are correct.

How would I turn this into a yes/no question in German?

You invert the subject and verb to get the verb in second position:
Stoppt der Fahrer das Auto?
(The object das Auto then follows.)

How do you form the simple past and present perfect of this sentence?

Simple past (Präteritum) of a weak verb adds -te + ending:
Der Fahrer stoppte das Auto.
Present perfect (Perfekt) uses the auxiliary haben + past participle (gestoppt):
Der Fahrer hat das Auto gestoppt.