I agree with Rusty.
Also, practice.
When I was learning German, I used "wo" as if it were "who" and "wer" as "where".
It's just a matter of practice. The more you make sentences with the difficult words, the more familiar they'll become. Write a lot, speak a lot, read a lot.
A conjugation manual is essential to me as a learner. Try this one:
Conjugación Verbal de la Lengua Española