Questions & Answers about Sein Mund ist klein.
Why is it sein and not seine?
The word sein means 'his'. We use sein without an extra ending here because the noun it describes, Mund (mouth), is masculine (der Mund) and is the subject of the sentence. If it were a feminine noun, like Nase (nose), you would need the feminine ending and say seine Nase.
Why doesn't the adjective klein have an ending?
In German, adjectives only get endings (like -e or -er) when they come directly before a noun, as in ein kleiner Mund (a small mouth). When the adjective comes after the noun and a verb like ist (is), it stays in its basic dictionary form, so it remains just klein.
Could sein also mean 'its' in this sentence?
Yes! The word sein is the possessive form for both masculine ('his') and neuter ('its') owners. Out of context, this could technically mean 'its mouth' (for example, if talking about a neuter animal like das Pferd, the horse), but 'his' is usually the most common translation.