The semantic differences of yào (要) and xiǎng (想) can be categorized according to sentence structure:
(1) Followed by a noun or a noun phrase
Both yào (要) and xiǎng (想) can be followed by a noun or a noun phrase. However, their meanings are not the same. yào (要) is followed by a concrete or an abstract noun or noun phrase. (Herzberg, Q., & Herzberg, L 2011). As shown in the following example, yào (要) means "to want something":
I want a cup of coffee.
In this case, yào (要) cannot be replaced by the singular word xiǎng (想). In other words, it is not grammatical to say:
However, it is grammatical to express "want something" by using xiǎng yào (想要) (Herzberg, Q., & Herzberg, L 2011).
As for single word xiǎng (想), when it is followed by a noun or noun phrase, it means "to miss’"(Liu, Yao, Bi, Ge and Shi 2009), such as examples shown below:
I miss home very much.
Similarly, the above examples cannot be replaced by the word ‘要’ as shown below:
(2) Followed by verb or verb phrase
Both yào (要) and xiǎng (想) can also be followed by a verb or verb phrase. When yào (要) is followed by a verb or a verb phrase, it refers to the meaning of "want to" (Liu, Yao, Bi, Ge and Shi 2009), indicating a strong desire, intention or plan as shown in the following examples:
I want to drink water.
I will attend a meeting in the U.S. next Monday.
In the first example, the sentence "我要喝水" can still be grammatical when the "要" is replaced by "想", it becomes:
I want to drink water.
Yet, the meaning is slightly different between two sentences. The degree of ‘emergency’ is much higher in "我要喝水".
In the second example, "到美國開會" is regarded as a duty, which is not the desire of the agent. Thus, when "想" is followed by a verb or a verb phrase, it contains the meaning as "would like to" (Liu, Yao, Bi, Ge and Shi 2009), indicating one's wish or plan:
I want to learn German.
(3) Following by adverbs indicating degree
There are some adverbs which indicate degree, such as hěn (很) and fēi cháng (非常). In this situation, only "想" can be used after those adverbs, and the use of "要" is ungrammatical (Teng 2011).
My sister want to eat candies badly.
Nevertheless, yī dìng (一定) can be followed by both "要" and "想", which means "must (do something)" and "must want to (do something)" :
The food in this restaurant is incredible. Everyone must try it.
The food in this restaurant is incredible. Everyone must want to go.