We've all experienced this. You type a word (a real, correctly spelled word) and your iPhone auto-corrects it to a different word. Anyone who has ever tried to spell 'fucking' and instead ended up saying 'ducking' knows exactly what I'm talking about.
Adding words to the iPhone dictionary is also great if you frequently write "technical" words (i.e. scientific words), if you want to use a word in a different language, or if you want to write an uncommon way.
Here's how you do it:
1. Open Safari
2. Tap the Google search bar in the top right corner
3. Type the word you want to add and click search
For example, if you work for a company called "SAF", but the iPhone always corrects it to "Sad", just search for "SAF" on Google one time.
That's it. There are other methods, such as adding the word as a name in your Contacts, but that takes more work and clutters your contacts.