Phonics is a key way to learn common letter combinations when spelling words.
You can read more here:
"Phonics is a method for teaching reading and writing the English language by developing learners' phonemic awareness—the ability to hear, identify, and manipulate phonemes—in order to teach the correspondence between these sounds and the spelling patterns (graphemes) that represent them."
For example: "ing" or "ph" usually have a certain sound, so that when spoken you can reasonably guess the correct letter combination.
Of course, some words borrowed from foreign languages will break those rules, but for common English it should be OK.