がち means "tend to", "be prone to", and the tendency is almost always unwelcome: forgetting, being late, falling ill. It goes on a verb's ます-stem or straight after a noun.
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He tends to be late for meetings.
、。
When I'm busy, I tend to forget breakfast.
、。
She was often ill as a child.
The verb goes in the ます-stem, not the dictionary form.
Before a noun, and as an adverb
がち behaves like a な-adjective: it takes before a noun and before a verb.
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In winter the weather is often cloudy for days.
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She began to speak a little hesitantly.
"reserved, hesitant" is one of the few where the tendency isn't bad.
がち, 気味 and やすい
がち says something happens again and again, as a habit. says a state is showing a little, right now. says something is easy to do or easily happens, and it can be good or bad.
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I tend to catch colds in winter.
、。
I've got a bit of a cold at the moment.
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This book is easy to understand.
- × この本は分かりがちだ → この本は分かりやすい
Usage notes
These go beyond a person's habits.
なりがち
"tends to become" is the commonest of all, for something that slides into a bad state.
、。
When you live alone, your meals tend to get irregular.
ありがち
means "typical", "all too common", often of a mistake or a cliché.
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This kind of mistake is all too common.