Necesito limpiar el cristal de la ventana para poder ver el sol.

Because after necesitar, Spanish normally uses another verb in the infinitive to mean need to do something.

• Necesito limpiar = I need to clean
• Necesita comer = He/She needs to eat

English uses to before the second verb; Spanish does not.

In Spain, el cristal often refers specifically to the glass pane, while la ventana means the whole window.

So:

• limpiar el cristal de la ventana = clean the glass of the window
• limpiar la ventana = clean the whole window

If the focus is on being able to see through it, mentioning el cristal is very natural.

Both can relate to glass, but they are not always used in exactly the same way.

In Spain:

• cristal is very common for a window pane or a drinking glass
• vidrio often refers more to the material glass

So in this sentence, cristal sounds especially natural.

For example:

• el cristal de la ventana = the window glass / pane
• una botella de vidrio = a glass bottle

De shows a relationship like of in English:

• el cristal de la ventana = the glass of the window

It tells you which glass you mean.

You need la because ventana is:

• feminine
• singular
• definite

So de + la ventana is the normal form.

Both are correct, but they are slightly different in nuance.

• para ver el sol = to see the sun
• para poder ver el sol = to be able to see the sun

Adding poder emphasizes ability. It suggests that the dirty glass is currently preventing you from seeing the sun clearly, and cleaning it will make that possible.

So para poder ver is a bit more explicit.

Yes, that is grammatical and natural.

But it changes the focus a little:

• limpiar la ventana = clean the whole window
• limpiar el cristal de la ventana = clean the glass part specifically

Also:

• para ver el sol = simpler
• para poder ver el sol = stresses being able to see it

So the original sentence is a bit more precise.

Spanish usually uses the definite article with things like the sun and the moon:

• el sol
• la luna

That is just the normal Spanish pattern. English often says the sun too, but in some contexts English drops articles where Spanish keeps them more consistently.

So ver el sol is the standard way to say see the sun.

Because grammatical gender belongs to each noun individually.

• el cristal is masculine
• la ventana is feminine

The gender of one noun does not change the gender of another nearby noun.

So:

• el cristal de la ventana
• not la cristal, because cristal itself is masculine

This is very common in Spanish: each noun keeps its own gender.

Because para expresses purpose, while porque expresses reason.

In the sentence:

• para poder ver el sol = in order to be able to see the sun

That is the goal or purpose of cleaning the glass.

If you used porque, you would be giving a reason instead:

• Necesito limpiar el cristal porque no puedo ver el sol
  = I need to clean the glass because I can't see the sun

So:

• para = purpose
• porque = cause/reason

Yes. Spanish word order is fairly flexible, especially with purpose phrases like this one.

For example, you could also say:

• Para poder ver el sol, necesito limpiar el cristal de la ventana.

This means the same thing. It just puts more emphasis on the purpose first.

The original order is probably the most neutral and straightforward.