Three characters with subtle differences:
干 is gān 'dry' and gàn 'do':
Common word: 干燥 gānzào (traditional Chinese 乾燥) is 'dry' (adjective).
你干嘛？ Nǐ gàn ma? 'What are you doing?'
千 Qiān is 'a thousand':
The top stroke has to start on the right.
于 Yú is 'at':
Note the hooked vertical stroke.
This preposition is commonly used with times in the spoken language; you will also see it used with places in the written language.