Probably the two most difficult words to tell apart are effect and effect. This is because their definitions are so similar. One is a noun, and the other can be a noun and a verb. They both have identical meanings about being the result of something else. The adjective forms of these words are effective and effective. They are probably just as difficult to determine which one to use in a sentence.
However, the most used version is effective. Affective means causing emotions or being emotional. Similar to effect, effective means to cause the desired result. Both are adjectives and are used in sentences usually right before a noun to modify or describe that noun. Sometimes, they may come after a linking verb.