It’s easy to see why people get confused between complement and compliment – they look similar and sound the same. Even their usages aren’t a million miles apart – but they mean two very different things. So, how can you remember which is which? How about this:
Complement means, according to the Oxford English Dictionary, the action of completing or fulfilling something. Each of the two items or functions looks better when they are used together.
For example, “those ruby earrings really complement your eyes”, or “I find Stinking Bishop to be the perfect complement to a large glass of Muscadet”. It links back to the word ‘complete’.
So if the meaning you want to convey has a sense of completion – two things combining to produce a good overall effect – the word you need is complement.
Compliment, on the other hand, means expressing admiration or praise for something.
For example, “John complimented Gareth on his exquisite taste”, “the policeman complimented the woman on her excellent driving,” or “you should always thank people if they pay you a compliment”.
So if you want to say something nice about someone, you need the ‘I’ version: compliment.
A good way to remember is to think of the classic compliment (or chat-up line. It’s a grey area), “what nice eyes (i’s) you have”.