Comparative

Add -er to the adjective, no matter how long the word is. Schnell gives schneller, klein gives kleiner, billig gives billiger, and even interessant gives interessanter. German never uses a separate word the way English uses more, so more interesting is simply interessanter, not mehr interessant.

Most one-syllable adjectives containing a, o or u take one: alt gives älter, jung gives jünger, groß gives größer, kurz gives kürzer, lang gives länger, warm gives wärmer, kalt gives kälter, stark gives stärker. Longer adjectives never do, so langsam stays langsamer. Treat the umlauting group as a short list to memorise.

Use als for than, when two things are unequal: Er ist älter als ich, Sie läuft schneller als er. Use wie only for as, when two things are equal, in the pattern so ... wie: Er ist so alt wie ich. Mixing them up is one of the most noticeable learner errors.

Six matter most. Gut becomes besser, viel becomes mehr, and gern becomes lieber — all completely different words, like good/better in English. Hoch drops its c to give höher, nah adds an umlaut to give näher, and teuer drops its e to give teurer. So Das Essen hier ist besser.

Yes. When the comparative stands before a noun it keeps the -er and then adds the normal adjective ending on top: ein schnelleres Auto, ein älterer Mann, die bessere Lösung. After sein and other linking verbs no ending is needed, which is why Mein Auto ist schneller has the bare form.