For people in adjective clauses there are four options: who, whom, that or nothing. Of these four options, whom is the most formal and is more commonly used in academic and professional writing.
For things there are only three options: which, that or nothing.
The example “Andy is a mechanic who you can trust” can be separated into two sentences: Andy is a mechanic. You can trust a mechanic. “A mechanic” is an object in the sentences. To combine two sentences, “a mechanic” in the adjective clause should be changed to who or that.
Who, that, which can be omitted in adjective clauses with object pronouns.
The movie which/that/- we watched yesterday was terrible.
The store manager whom/who/that/- they asked was rude to them.
“Paddington started. He put the suitcase guiltily behind himself and sniffed. He had several expressions which he kept for emergencies. There was his thoughtful expression, when he stared into space and rested his chin on a paw.” [1, p.41]
“He examined carefully the piece of green cardboard which he held in his paw. “It doesn’t seem much to get for tenpence,” he said. After all the lovely whirling and clanking noises the ticket machine had made, it did seem disappointing. He’d expected much more for his money.” [1, p.42]
Sources:
- Bond, M. (1992). A Bear Called Paddington. HarperCollinsPublishers.