Foronya hii ni safi, kwa hiyo nitaweka mto wangu ndani yake.

Because foronya belongs to noun class 9 in the singular, and the class 9 form of this is hii.

So:

• foronya hii = this pillowcase

If you made it plural, you would use hizi:

• foronya hizi = these pillowcases

This is one of the first big things English speakers have to get used to in Swahili: words like this, that, my, and some adjectives change according to noun class.

Here, ni is the copula, meaning is/are.

So:

• foronya hii ni safi = this pillowcase is clean

Also, safi is a descriptive word that usually stays the same rather than changing form for different noun classes. That is why you do not see an agreement form attached to safi.

Kwa hiyo means so, therefore, or for that reason.

It connects the first idea to the result:

• The pillowcase is clean
• therefore / so
• I will put my pillow inside it

It is a very common way to link cause and result in Swahili.

Nitaweka breaks down like this:

• ni- = I
• -ta- = future marker
• weka = put / place

So:

• nitaweka = I will put

The dictionary form of the verb is kuweka = to put / to place.

In Swahili, possessives normally come after the noun.

So:

• mto wangu = my pillow

not:

• wangu mto

The possessive stem is -angu = my. It changes its form depending on the noun class of the noun it describes. With mto, it appears as wangu.

Because the object mto wangu is already stated explicitly after the verb.

So Swahili can simply say:

• nitaweka mto wangu = I will put my pillow

An object marker is often unnecessary when the object is clearly named, especially with inanimate things. English speakers sometimes expect every object to be marked on the verb, but Swahili does not always do that.

Ndani means inside or interior.

Yake here means of it / its.

Together:

• ndani yake = inside it

This is a very common Swahili pattern with location words:

• juu yake = on it
• chini yake = under it
• mbele yake = in front of it / him / her
• ndani yake = inside it

Yes, exactly. Yake is a possessive form that can mean his, her, or its, depending on context.

Here it refers back to foronya, which is a non-human thing. So in this sentence, yake is understood as its:

• ndani yake = inside it

That may feel strange at first to an English speaker, but it is very normal in Swahili.

Yes, absolutely.

These are both natural:

• nitaweka mto wangu ndani yake
• nitaweka mto wangu ndani ya foronya hii

The second version repeats the noun and is more explicit. The version in your sentence uses yake to avoid repetition.

A very literal breakdown is:

• Foronya hii = this pillowcase
• ni safi = is clean
• kwa hiyo = so / therefore
• nitaweka = I will put
• mto wangu = my pillow
• ndani yake = inside it

So the whole sentence is structured very close to English:

• This pillowcase is clean, so I will put my pillow inside it.