大: big; large; great. JLPT N5; 3 strokes.
大 means 'big' with regard to size, quantity or importance; as a prefix it strengthens the meaning of the word, as in 大好き (だいすき, to like very much).
On readings: ダイ in 大学 (だいがく, university), 大事 (だいじ, important), 大丈夫 (だいじょうぶ, okay, all right). タイ in 大切 (たいせつ, precious, important), 大変 (たいへん, very; tiring), 大会 (たいかい, tournament, gathering), 大使 (たいし, ambassador), 大抵 (たいてい, usually). There is no simple rule for choosing between ダイ and タイ: the reading must be learned word by word.
Kun readings: おお in 大きい (おおきい, big), 大勢 (おおぜい, many people), 大雨 (おおあめ, heavy rain), 大阪 (おおさか, Osaka). 大いに (おおいに) means 'very, greatly'.
Special readings: 大人 (おとな, adult) and 大和 (やまと, old name of Japan) are read as whole words.
Remember: 大きい and 大きな have the same meaning, but 大きな is only used before a noun (大きな木, a big tree). The opposite is 小さい (ちいさい, small).
この犬は大きいです。
kono inu wa ookii desu.
This dog is big.
Follow the steps in order. Each new stroke is highlighted.
Stroke paths: KanjiVG, CC BY-SA 3.0. Sequential diagrams derived from the original paths.