I decided don't tell him about it. Де помилка?

Помилка в тому, що після decided потрібно вживати to + infinitive.

❌ I decided don't tell him about it.

✅ I decided not to tell him about it.

Правило:

decide to do something — вирішити щось зробити.

Заперечення: decide not to do something.

Приклади:

I decided to stay at home.

I decided not to tell him about it.

She decided not to go to the party.

Українською:

I decided not to tell him about it. — Я вирішив не розповідати йому про це.