The nearby people questioned why the car (that was parked in front of the gate) remained there for 3 hours when other cars were usually parked in the parking lot.

Correct sentence : The nearby people questioned why the car that was parked in front of the gate remained there for 3 hours when other cars were usually parked in the parking lot.

The phrase "that was parked in front of the gate" is restrictive, as without it the sentence is unclear (To which car are people referring? Where is the car?) and the meaning is lost. In general, a comma does not precede the word "that," as the phrase that follows "that" generally restricts or limits the thing that it modifies. In such a case, the clause is restrictive and does not need to be punctuated with commas, parentheses, or dashes.

Until the principal dismissed a student from the class there were twenty students.

Correct answer: Until the principal dismissed a student from the class, there were twenty students.

The introductory clause must be followed by a comma.

Cucumbers are used in many different types of cooking, and are grown around the world in both fields and greenhouses.

Correct sentence: Cucumbers are used in many different types of cooking and are grown around the world in both fields and greenhouses.

When the subject is the same in both clauses and is not repeated in the second clause, do not use a comma.

Throughout this year, the music of David was far more popular among audiences in Australia than John Peterson.

Correct sentence: Throughout this year, David’s music was far more popular among audiences in Australia than John Peterson’s music.

In the above sentence, music (thing) is being compared to John Peterson (person). In order to make the sentence correct, we must compare music to music.