てもらう and how it's used with particle に


I was watching this [video](https://www.youtube.com/watch?v=lp-2PNSANJ8) about
てもらう and てくれる, and I don't really understand why てくれる always uses が, and てもらう
always uses に to mark the person who does something for the speaker,
especially about に and てもらう. For example, what's the difference between the
following sentences?

> さくらがパーティーに誘ってもらった

> さくらにパーティーに誘ってもらった

why does てもらう always use に in the video? And not が？




もらう ("to receive") is a verb that can take up to three particles (i.e.,
[trivalent](https://en.wikipedia.org/wiki/Valency_\(linguistics\))). A **が** B
**に** C **を** もらう means "A (gratefully) receives C from B".

> 彼女 **は** 母 **に** 指輪 **を** もらった。  
>  She received a ring from her mother.  
>  (が is replaced to は because it's the topic of the sentence)

が (or は) marks the subject who is the receiver, and に (or から) marks the giver.
These rules won't change when もらう is used as a donatory subsidiary verb (i.e.,
when the given thing is a "favor" instead of a tangible object). Therefore:

> さくら **が** パーティーに誘ってもらった。  
>  Sakura was invited to a party (by someone, and she was grateful).  
>  (Read this as though Sakura received an imaginary invitation.)

> さくら **に** パーティーに誘ってもらった。  
>  [I/Someone] was invited to a party by Sakura (and I/he/she was grateful).

Of course you can use both が (は) and に and say something like this:

> 太郎が花子にパーティーに誘ってもらった。  
>  Taro was invited to a party by Hanako (and Taro was grateful).




もらう ("to receive") is a verb that can take up to three particles (i.e.,
[trivalent](https://en.wikipedia.org/wiki/Valency_\(linguistics\))). A **が** B
**に** C **を** もらう means "A (gratefully) receives C from B".

> 彼女 **は** 母 **に** 指輪 **を** もらった。  
>  She received a ring from her mother.  
>  (が is replaced to は because it's the topic of the sentence)

が (or は) marks the subject who is the receiver, and に (or から) marks the giver.
These rules won't change when もらう is used as a donatory subsidiary verb (i.e.,
when the given thing is a "favor" instead of a tangible object). Therefore:

> さくら **が** パーティーに誘ってもらった。  
>  Sakura was invited to a party (by someone, and she was grateful).  
>  (Read this as though Sakura received an imaginary invitation.)

> さくら **に** パーティーに誘ってもらった。  
>  [I/Someone] was invited to a party by Sakura (and I/he/she was grateful).

Of course you can use both が (は) and に and say something like this:

> 太郎が花子にパーティーに誘ってもらった。  
>  Taro was invited to a party by Hanako (and Taro was grateful).

