I can NEVER teel if it should be effect or affect. What are some ways that help you understand the differance?
First, they're both stressed on the second syllable, so the A or E is pronounced shwa (/ə/), regardless.
I.e, both words are pronounced /ə'fɛkt/.
Second, the word /ə'fɛkt/ that is a verb is spelled with A, while the one that is a noun is spelled with E. It is not a surprise that they are so similar, since they are related in meaning. What's unusual is spelling them differently this way.
- This depression will affect my investments.
- This depression will have an effect on my investments.
In (1), affect is a verb; in (2), effect is a noun, part of the idiom
have ___ effect on,
which means the same thing as affect.
Third, there are also two other -- rare, and normally idiomatic -- uses of the spellings affect and effect,
but they're easy to distinguish from the normal ones described above.
One, the noun affect, is stressed on the first syllable, so that the vowel is not reduced to shwa, but rather fully pronounced: /'æafɛkt/. It also has a very restricted meaning, as a clinical technical term for display of emotion.
- He has very low affect; pay attention or you might miss something important.
The other use, the verb effect, usually has a secondary stress on the first syllable, so that the vowel is often if not always /i/, and not shwa. In meaning, it also has a limited distribution, being always transitive, and used mainly in the idioms effect an escape, effect a solution, and the like. It's archaic and somewhat formal, the sort of thing that shows up in 19th-century newspapers and court proceedings.
- The burglar effected his escape by means of the bathroom window.
Nowadays we would use the verb escape instead of having to use effect or something similar.