Distinguish wanting an object from wanting to do something.
The rule
ほしい expresses the desire to get one thing: noun + がほしいです. To want to accomplish an action, we instead use the verbal form in たい.
Examples
あたらしいかばんがほしいです。
I'd like a new bag.
みずがほしいです。
I want some water.
Remember
- noun + がほしい; base in ます + たい for one action.
- Distinguish wanting an object from wanting to do something.
Common mistake
Use ほしい directly after a verb instead of たい.