(someone) is an expert on (a topic)

Make sure to use "on" when you're telling the topic that a person is an expert on:

She's an expert on organic farming.

You can also call someone "a ___ expert", but it doesn't work with longer phrases.

Tom is a social media expert.

It would be a little strange to say "Tom is a social media marketing expert" because that's too long.

This phrase appears in these lessons: