1Very; greatly; extremely — an adverb that strongly intensifies an adjective or verb. Slightly more formal than とても.
今日は大変寒いので、暖かくしてお出かけください。
It's extremely cold today, so please bundle up before you go out.
先日は大変お世話になりました。
Thank you very much for your help the other day.
この度は大変ご迷惑をおかけしました。
I'm terribly sorry for the trouble I've caused you.
試験の結果を聞いて、大変驚きました。
I was greatly surprised when I heard the exam results.
2Immense; enormous; great — describing something of very large size, degree, or significance.
大変な努力の末、彼はついに夢を叶えた。
After enormous effort, he finally made his dream come true.
このプロジェクトには大変な予算がかかった。
This project cost an enormous amount of money.
彼女は大変な読書家として知られている。
She is known as a great reader.
3Serious; grave; dreadful; terrible — indicating an alarming or critical situation. Often used as an exclamation 「大変！」 to express shock.
大変！電車に財布を忘れてしまった！
Oh no! I left my wallet on the train!
このまま雨が降り続いたら、大変なことになる。
If the rain keeps falling like this, it'll turn into a serious situation.
子どもが階段から落ちて、大変な騒ぎになった。
The child fell down the stairs, and it caused a real commotion.
4Tough; difficult; hard; demanding — describing something that takes a lot of effort or is exhausting. The most common everyday usage.
小さい子どもが二人いると、毎日が大変だ。
With two small children, every day is tough.
引っ越しの準備って、本当に大変ですね。
Preparing for a move is really exhausting, isn't it?
毎日残業で大変そうだね。
Working overtime every day looks really rough.
一人で全部やるのは大変だから、手伝うよ。
Doing it all alone would be too much — let me help you.
5Archaic noun meaning a major incident or disaster. Essentially unused in modern Japanese; found only in classical texts.