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?

German verbs change with the subject:

ist is 3rd‑person singular (he/she/it).
sind is 1st‑ and 3rd‑person plural (we/they).
bist is 2nd‑person singular (you).
Since Er is “he,” you use ist.

How do I turn Er ist pünktlich into a yes/no question?

Move the verb to the first position (V‑2 rule for questions):
Ist er pünktlich?

How would I express the opposite, i.e. say “He is not punctual”?

You have two main options:
1) Use nicht to negate: Er ist nicht pünktlich.
2) Use the antonym unpünktlich: Er ist unpünktlich.

When do adjectives in German require endings?

Adjectives get endings when they stand before a noun (attributive use). For example:

ein pünktlicher Mann (a punctual man) → -er for masculine nominative after ein.
But after sein, you leave them uninflected.

Can pünktlich also function as an adverb, and if so, how does that work?

Yes. When modifying a verb, pünktlich is used adverbially without change:

Er kommt pünktlich. (“He arrives on time.”)
Same form, different function.

What’s the difference between pünktlich and rechtzeitig?

pünktlich = “exactly on the agreed time.”
rechtzeitig = “in good time,” often slightly before a deadline or sufficiently early.