Formidable describes a foe you're slightly afraid of, but formative describes what formed you. Perhaps a formidable gym teacher told you to get the lead out during your formative years in grade school, and now you're a world-class athlete. (Or a bookworm, depending on how you react to formidable foes.)

If you're a software designer, Microsoft is a formidable opponent. Such an enemy is one you dread, or perhaps respect or are awed by, because of its power, size, or capabilities. If you have a formidable vocabulary, you have a lot of words to choose from:

"A really formidable fighter can be worth more than a horse," he said. (New York Times)

Temple turned out to be more formidable than most expected. (Washington Post)

Heath Shuler is taking on an even more formidable opponent: House Speaker Nancy Pelosi. (Knoxville News)

Something formative shapes or forms a person or thing. Your early years are generally considered formative because they greatly influence who you are. Let's form an idea from some examples:

Psychologists have always stressed the formative influence of parents, but siblings have been studied less. (New York Times)

Marc Webb's film explores the formative years of Peter Parker, previously played on screen by Tobey Maguire. (BBC)

Naturally, he has formative memories built around the intersection of the team with his life. (Seattle Times)

Remember: your formative period was when you were learning to become a formidable foe for words with unruly definitions. And mean gym teachers.