Hi Terry Just to say the phrase would be I'm planning to enter a race, I'm planning to enter some races, (not enter for). I run really well so I'm planning to enter some races. I think I run really well so I'm planning to enter some races. He is going apply to join in a race or join in races in the future. He has entered several competitions, and has won many gold medals. (In the past or recent past he has taken part in competitions).

Terry, Gillian is right. The problem arises when sometimes you see "enter in". How should we understand this? See definition 2.2 in the Oxford Dictionaries: 2.2 Register as a competitor or participant in a tournament, race, or examination: Example: They won every race they entered. Example: The horse was entered in the Martell Cup at Aintree. You can enter the race, but you can't enter the Cup. So you need a preposition "in". Some dictionaries give "for" as the preposition. "In" is the correct preposition. Alternatively, you can say, "I am competing for the Martell Cup."