Hi, there are following examples that when you use "wa" and "ga" and "o" in a conversation. "This book is interesting." 「この　本　は　おもしろい。（kono hon wa omoshiroi.）」 "That apple was rotten." 「あの　リンゴ　は　くさって　いたよ。（ano ringo wa kusatte itayo.）」 "The book is burning." 「本　が　もえて　います。（hon ga moete imasu.）」 "The baby is crying." 「あかちゃん　が　ないて　います。（akachan ga naite imasu.）」 "Put your books away." 「本　を　かたづけ　なさい。（hon o kataduke nasai.）」 "He broke his promise." 「かれ　は　やくそく　を　やぶった。（kare wa yakusoku o yabutta.）」 Good luck!

は is commonly used to make something as topic, while が is used mainly to describe an object. :)

yes, that's basic and interesting to know. im commenting to follow the replies.