Grateful is what you feel towards someone who has done you a kindness. The neighbour who took your parcel in, the stranger who found your phone and rang the last number in it — you are grateful to them.
The grammar matters: you are grateful TO a person, grateful FOR a thing, and grateful that something turned out as it did. It runs warmer and more personal than thankful, which leans towards relief — thankful the plane landed, grateful to the friend who drove you to the airport. In letters and emails it goes formal: I would be grateful if you could send the file by Friday is a polite instruction, not a feeling. The noun is gratitude, and ungrateful is the sharp opposite, the child who opens a present and asks what else there is. One trap: grateful describes the person who receives, never the gift — the person is grateful, the kind act is not.
1adjectivegratefulFeeling or showing thanks for something kind that someone has done.
“She was grateful to the nurse who sat with her all night.”
synonyms: thankful, appreciative
2adjectivepoliteUsed in polite formal requests to say that something would be welcome.
“I would be grateful if you could reply before Monday.”
Too few real examples of this word to estimate which sense is commonest.
Also accepted in games: thankful
Grateful is grate plus -ful, built on an English adjective grate that has since died out, meaning 'pleasing'. That came from Latin grātus — agreeable, dear, thankful — and further back from the Proto-Indo-European root *gʷerH-, 'to praise, to express approval'. What is grateful was first simply what pleased you.
Proto-Indo-European
*gʷerH- — to express approval, praise; to elevate
Latin
grātus — agreeable, pleasing; beloved, dear; grateful, thankful
English, today
grateful
From grate + -ful. Grate is a Latin grātus (“agreeable, pleasing; beloved, dear; grateful, thankful”), ultimately from Proto-Indo-European gʷerH- (“to express approval, praise; to elevate”).
Etymology from Wiktionary, CC BY-SA