What is the subtle difference between 志愿, 意愿, and 心愿? I noticed many Chinese words have the same or similar meanings but they are not exchangeable. In many cases it is hard to decide which one to choose. Can anyone tell the subtle difference between 志愿, 意愿, and 心愿?

For example, I never heard anyone say "你的生日意愿是什么". Usually you heard "你的生日心愿是什么". But both 意愿 and 心愿 are "wish" in English.
Consider, in English, a single word can have more than one meaning, and two different words may have a function overlapped.
For example:

'Goal' can be replaced by 'wish' in some instances - e.g. &quot;My Goal(wish) is to become a doctor&quot;

'Dream' can be replaced by 'wish' in some instances - e.g. &quot; My Dream(wish) is to become a doctor&quot;

'Will' can be replaced by 'wish' in some instances - e.g. &quot; To become a doctor is my will(wish)&quot;


As for the difference between 志愿, 意愿 and 心愿

志愿 mainly mean: Goal; Dream

意愿 mainly mean: Will; Desire

心愿 mainly mean: Desire; Wish


All three above can mean 'Wish'

我的志愿（愿望）是成为一名医生 - My goal (wish) is to become a doctor
我的意愿（愿望）是成为一名医生 - My will (wish) is to become a doctor
我的心愿（愿望）是成为一名医生 - My desire (wish) is to become a doctor

The important thing is, it all depends on the context.
The best term for &quot;Wish&quot; in &quot;What is your birthday wish?&quot; is '愿望'  As in &quot;你的生日愿望是什么&quot;
Yes, 愿望 mainly mean 'wish' (n) but in some instances, you can translate it to 'desire, hope or dream'
Some more tips:

Use 志愿 if the wish is also an achievable goal (志 means goal, 志愿 = a wish you aim to achieve)
Use 意愿 if the wish is a determined one (意 means mind, 意愿 = a wish you have made up your mind to fulfill)
Use 心愿 if the wish is also a deep desire or a life long dream (心 means heart, 心愿 = a wish comes from your heart)