There are always three suggested words above the keyboard as you type, and the one on the left is exactly what you type. Tap that and it will put in your typing and learn it as a word for the next time you type it.
No mattter how many times you go back and type the word? In that case, once you finish typing the word and ensure you have it spelled correctly, and BEFORE you hit the space or punctuation keys, tap the word as you typed it in the predictions selections above the keyboard. That adds it to your personal dictionary and will recognize it then and in the future.
The flip side of this is that you can long press any word in the predicitions to no longer have it suggested (for excample if you saved a typo or misspelled someone's name previously).
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Swiftkey sucks. I will never get past the fact that you cannot quickly text what you want how you want. Their corrections are aggressive and cannot be turned off.
Swiftkey settings, advanced, spacebar completion mode, "always insert a space".