adjective
BAD : I'm very thankful to you for giving me this opportunity.
GOOD : I'm very grateful to you for giving me this opportunity.
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thankful = happy and relieved that something (good) has happened: 'We should all be thankful that nobody was hurt.'
grateful = full of thanks: 'Dr Cameron has received dozens of letters from grateful patients.' 'We'd be grateful if you could let us have your views on this matter.'