Learn Hardcore German: Er ist pünktlich.

pünktlich is an adjective used predicatively. That means it follows a linking verb (here ist) and describes the subject without taking any ending.

Why doesn’t pünktlich take an ending in this sentence?

In German, adjectives directly after a form of sein (to be) remain uninflected. They never get adjective endings when they stand as predicate adjectives.

Why is pünktlich written in lowercase while Er is capitalized?

German capitalization rules say only nouns—and the first word of a sentence—are capitalized. pünktlich is an adjective, so it stays lowercase.

How do you pronounce the ü in pünktlich, and why is there ck instead of just k?

The ü is a front rounded vowel: try saying English “ee” (as in “see”) while rounding your lips.
The ck spelling signals a short preceding vowel (short ü). It’s pronounced like a hard k.

Why is the verb sein here ist instead of sind, bist or another form?