Learn Swedish: Den äldre herren drack sällan kaffe. - The older gentleman rarely drank coffee.

Not necessarily. This is called an absolute comparative. In Swedish, using the comparative form like äldre (older) often describes a moderate degree of the adjective without a direct comparison. Here, Den äldre herren simply means the elderly gentleman. It is actually considered a bit softer and more polite than calling him Den gamla herren (the old gentleman).

Why doesn't the adjective end in -a since it follows the definite article Den?

You are right to expect an -a ending for an adjective in a definite phrase. However, comparative adjectives in Swedish are an exception. They always end in -e or -re (like äldre), and they never change their endings to match gender, plurality, or definiteness.

Why does sällan (rarely) come after the verb drack?

In a main clause, Swedish strictly follows the V2 rule, meaning the verb is the second piece of information. Sentence adverbs that modify the whole sentence, like sällan (rarely), ofta (often), or inte (not), typically come right after that verb.

Why is the word for gentleman herren instead of herr?