Translation:This bus is big.
この (kono): this
バス (basu): bus
は (ha): particle which indicates the topic
大きい (ookii): big
です (desu): to be
I thought 大 was pronounced "dai" why is it "oo" in here? How do I know when the pronunciation changes? Does it only apply with the context?
Kanji do not have singular pronunciations. Kanji are conceptual, not literal. Pronunciations are based not on the kanji themselves but on their context. There are usually at least 2 pronunciations for each character and often 4 or 5. The pronunciation often depends on whether the the character is connected with other kanji as a compound word (usually on'yomi reading) or is accompanied by kana (usually kun'yomi reading).
Japan has a more native language represented by the word kun'yomi as well as an imported language based on Chinese reading referred to as on'yomi. There are usually multiple ways of expressing the same concept, which might differ in pronunciation even if they all include a common kanji element such as 大.
大 can be pronounced as 'tai', 'dai', 'oo', 'o' (大人）, and possibly another that I'm forgetting. It is good practice to memorize a kanji in context but to always have in mind that this is 'a' reading, not 'the' reading.
Thank you so much! I haven't gotten many kanji and I see other people use them and I was confused