Ikiwa radi itaonekana tena, tutafunga madirisha.

Ikiwa means if.

It introduces the condition in the first clause:

Ikiwa radi itaonekana tena = If ... appears again

For a learner, the easiest way to remember it is:

ikiwa + clause = if + clause

Yes, often you can.

A very common everyday alternative is:

Kama radi itaonekana tena, tutafunga madirisha.

Both ikiwa and kama can mean if.
In many contexts, ikiwa can sound a bit more formal or more written, while kama is very common in speech.

In English, we usually say:

If it appears again, we will close the windows.

English uses a present form after if.

Swahili often uses the future marker -ta- in both clauses:

• itaonekana = it will appear / it will be seen
• tutafunga = we will close

So Swahili structure may look more like:

If it will appear again, we will close the windows

That is normal Swahili grammar, even though the most natural English translation uses present tense in the if clause.

Itaonekana can be broken into:

• i- = subject marker for it
• -ta- = future marker
• -onekana = appear / be visible / be seen

So:

itaonekana = it will appear / it will be seen

This verb is related to -ona meaning see.
So kuonekana literally has the idea of being seen, and from that it can also mean appearing or being visible.

Because the subject is radi, and radi takes this agreement marker.

In Swahili, verbs agree with the noun class of the subject.
Here, radi belongs to a class that uses i- as the singular subject marker in this tense.

So:

• radi = the subject
• i- = the matching subject prefix on the verb

This is why you get:

radi itaonekana

not something like radi ataonekana or radi litaonekana.

Tena means again.

In this sentence:

itaonekana tena = will appear again

It usually comes after the verb or verb phrase it is modifying.

So the order here is very natural:

• itaonekana tena = appear again
• tutafunga tena would mean we will close again

Tutafunga breaks down like this:

• tu- = we
• -ta- = future marker
• -funga = close / shut

So:

tutafunga = we will close

This is a very common Swahili pattern:

• nitafunga = I will close
• utafunga = you will close
• atafunga = he/she will close
• tutafunga = we will close

Madirisha is the plural of dirisha, meaning window.

So:

• dirisha = window
• madirisha = windows

This is a noun class pattern in Swahili. Many nouns in this class form the plural with ma-.

Also, Swahili does not use articles like the or a.
So madirisha can mean windows or the windows, depending on context.

There is no separate word for will.

In Swahili, future meaning is built into the verb with the marker -ta-.

So:

• itaonekana: the -ta- means will
• tutafunga: the -ta- also means will

That means Swahili often packs information into one verb word that English would express with separate words.

Yes.

You can also say:

Tutafunga madirisha ikiwa radi itaonekana tena.

That still means the same thing. The difference is mostly one of emphasis or style.

Putting the if clause first is very common, especially when you want to present the condition before the result.