想 xiǎng 要yào 想要 xiǎng yào can all be used to describe the action of ‘to want to’ in Chinese. However, there are some differences between them, especially in terms of the degree of one’s desire.
Scenario 1 In a restaurant, Charles is the one who’s paying, Aya is Charles’ girlfriend…
wǒ xiǎng hē chá, kě yǐ ma
Chelsea: 我想喝茶，可以吗？ I want to (would like to) drink tea if that’s okay?
In such case, because Chelsea is the guest, she wants to be polite by saying she would like to have a cup of tea, but the degree of want is not very strong. It would be nice to have a cup of tea, but it is okay if she can’t have one.
qīn’ài de, xiǎng yào yī hú chá
Aya： 亲爱的，Chelsea 想要一壶茶。Honey, Chelsea wants a pot of tea.
Here, Aya is delivering the message from Chelsea to Charles. The desire of 想要 is a degree higher than 想。
fú wù yuán, yào yī hú chá
Charles： 服务员，要一壶茶。Waiter／waitress，pot of tea.
Charles then ask the waiter for a cup of tea. Because this is no longer a desire but a demand, so Charles uses 要 to the waiter.
Note that apart from ‘to want to’, 想 also has the meaning of ‘to miss’, and ‘to think’ ，therefore, there has to be a verb behind 想 in order to distinguish ‘to want to’ and its other meaning. For instance, if instead of saying ‘我想喝茶’， Chelsea says ‘我想茶’， then the meaning of the sentence would be changed to ‘I miss tea’. If the sentence changes to ‘我想我要喝茶’， then the meaning would be ‘I think I want to drink tea’ .
Let’s take a look of a different example:
wǒ xiǎng tā
我想他。 I miss him
wǒ yào tā
我要他。 I want him (I am going to have him)
wǒ xiǎng yào tā
我想要他。 I want him (I wish to have him)
Also note that 我要他 and 我想要他 both have some sexual reference as in ‘I want to have sex with him’…. so make sure to use it appropriately :P!