satooya ni naru.

It is read さとおやになる.

• 里親 = さとおや
• に = に
• なる = なる

So the whole sentence is さとおやになる.

The spaces are just there to help learners see the parts more clearly.

In normal Japanese writing, this would usually be written as:

里親になる。

Japanese normally does not put spaces between words the way English does.

Here, に marks the result or new state.

The pattern is:

noun + に + なる = to become + noun

So:

• 里親になる = to become a foster parent

In other words, に shows what someone changes into.

Because なる means to become, while する means to do or to make.

• 里親になる = become a foster parent
• 里親にする would mean something more like make someone/something into a foster parent, which is not the normal idea here

So if the meaning is that a person changes status and becomes a foster parent, なる is the correct verb.

Usually, 里親 means foster parent.

That is the normal word for someone who takes care of a child as a foster parent. If you specifically mean a legal adoptive parent, Japanese often uses different wording, such as 養親 or expressions involving 養子縁組.

So for learners, the safest understanding is:

里親 = foster parent

Because Japanese often leaves out the subject when it is already understood from context.

So 里親になる。 could mean things like:

• I will become a foster parent
• She becomes a foster parent
• They are becoming foster parents

The exact subject depends on the situation. Japanese does this very often, and it is completely normal.

This is the plain dictionary form, also called the plain non-past form.

That form can be used for several things depending on context, such as:

• a general statement
• a future action
• a dictionary-style example

So 里親になる。 can mean something like:

• become a foster parent
• will become a foster parent

If you want other common forms:

• polite: 里親になります
• past plain: 里親になった
• past polite: 里親になりました

Yes, it can be a complete sentence, but it is very short and context-dependent.

In textbooks or vocabulary examples, short sentences like this are common because they clearly show the grammar pattern. In real conversation, people often say something a little fuller, such as:

• 将来、里親になりたいです。 = wanting to become a foster parent in the future
• 彼は里親になりました。 = he became a foster parent

So 里親になる。 is grammatical and natural, but by itself it sounds a bit bare unless the context is already clear.

Not exactly.

It can be tempting to match に with English to, but that is not really what is happening here. In noun + に + なる, に is marking the state or role that someone ends up in.

So it is better to learn the whole pattern:

X に なる = to become X

That will help more than trying to translate に word-for-word each time.