Use 想 in case you * missing someone 我想你了 I'm missing you * want to do something 我想去吃午饭 I want to have lunch 我想去上学 * thinking 我在想昨天那家餐馆 I was thinking about the restaurant we've been 你在想什么？ What are you thinking about?

要 is more certain than 想

When you put these two words together, you got 想要 , which can be same as 想 or 要 according to various scenario. But will never means "thinking 想".

Synonymous, hard to define. Use 要 in case you wanted to, need to , or had to. For example: 我要走了 I have to leave. 我要吃东西了 I have to eat something 我要买点东西 I need to buy something 他要我帮他拿东西 he asks me to bring him something something/situation you can't refuse, or deny, such as 我要死了 I'm going to died 我要被他气疯了 you need something, to get 他从篮子里要了个苹果 He grabbed an apple from the basket