Simply put, it's the result of a historical sound change that took place tens of hundreds of years ago. It's the same reason we say "mouse" and "mice" instead of "mouse" and "mouses" or why Germans say singular Fuß "foot" but plural Füße "feet".
If you want to know more, you can check out this Wikipedia article: https://en.wikipedia.org/wiki/Germanic_umlaut But it probably won't be of any use unless you have some background in linguistics.
But in practice, it's not something you can predict... at least, not as far as I know. You're just gonna have to memorize it. Sorry :/
If you're having trouble with the letter "æ", you have a few options.
If you're on a mobile device, you can...
- Try to hold down "a", and see if "æ" appears as an option.
- Install an international keyboard.
- Install a Norwegian keyboard.
If you're on a computer, you can...
- Click the letters provided to you below the input field.
- Install a Norwegian or international keyboard.
- Learn the ALT-codes for the Norwegian characters.