"He fears the dog."
Translation:Li timas la hundon.
To help me remember, I like to look at the etymology of a word or think of a similar word in a language I know. In this case, the mnemonic I am using is that timas makes me think of timid, and timid people are often afraid. Hopefully thinking of something like that would help you.
How do you remember any Esperanto words? Constant practise, I suppose.
Perhaps for this word, Timeo Danaos et dona ferentes may help, if that's a saying that you know? (I fear the Danaans/Greeks even if they bring gifts - https://en.wikipedia.org/wiki/Timeo_Danaos_et_dona_ferentes )
Esperanto timi is related to this Latin word timeō.