particle
too, to (infinitive marker)
A1
Zu is a modal particle used two ways: as an infinitive marker ('zu' + infinitive, e.g., zu gehen) and as 'too' meaning 'excessively' (zu teuer → too expensive). It’s not a preposition, so it doesn’t govern case; it appears unstressed when marking infinitives or qualifying adjectives/adverbs.
Examples
Als die Musik leiser wurde, begann die Gruppe zu sprechen, weil die Fragen wichtiger waren.
When the music got quieter, the group began to speak because the questions were more important.
Das ist zu teuer.
That's too expensive.
Er hat vergessen, zu kommen.
He forgot to come.
Details
Mnemonics
Picture a small arrow pointing 'to' something with the label 'zu' — the infinitive marker.
Sounds like 'zoo' — imagine adding 'too' at the zoo: 'zu' = 'too'.
Notes
'zu' as a particle can mark the infinitive (zu kommen) or act as an intensifier meaning 'too' (zu teuer). Context decides the function.