Intervene is stepping in to help. Interfere is getting involved where you are not wanted.
Both words describe someone joining in with a situation that was not theirs to start with, so children often pick whichever one comes to mind first. The difference is whether anyone is glad they did.
When two Year 5s start shoving each other over a football and the teacher steps between them, the teacher has intervened. Things were going wrong, and stepping in made them better.
When your little brother keeps grabbing the spanner while you are fixing your bike puncture, he is interfering. Nobody asked for his help, and he is making the job harder. A quick test: intervene is usually something you would thank someone for; interfere is something you would ask them to stop.
To get involved in something that is not your business, often making it worse.
Similar: meddle, intrude, disrupt
To step in to stop trouble or help sort something out.
Similar: interfere, interrupt, mediate
Comprehension questions often ask what a word tells you about a character or about the writer's view. A character who interferes is being criticised; one who intervenes is usually being praised. Swapping them flips the meaning of the sentence.
Definitions and example sentences on this page are the ones a child sees inside Wordrealm, taken from our list of over 1,100 words. It is free to browse, with nothing to sign up for.
Plain-English guides to each test, from the councils’ own pages: Kent Test · Medway Test · Bucks Secondary Transfer Test