These two adjectives are very often confused by English students because they are similar in meaning; however, they are used differently.
We use "thankful" to mean that we are relieved that something unpleasant of dangerous didn't happen. It has a suggestion of thanking destiny rather than people.
- We should be thankful that it didn't rain during the match.
- After the tsunami, we felt thankful to be alive..
We use "grateful" when we want to refer to how we feel when somebody is kind to us or helps us in some way.
- I'd be very grateful if you could help me.
- I'm grateful for you giving me a chance to put things right.
In the picture below, you will find some more examples that will help you to better understand how to use these two adjectives.
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