We write what we say. Non-native English speakers tend to write English words as they pronounce them in their native accent. E.g., ‘morden’ is incorrect and ‘modern’ is the correct spelling. Pronounce words correctly.
Say it out loud. This is one great way to avoid spelling mistakes. When you read words the way you would pronounce them, you will easily find the mistake.
Use spell checker applications. These will help you identify spelling mistakes while you practice your writing tasks.