Kad pada kiša, uvijek stavim kapuljaču na glavu.

Here kad means when, but in this kind of sentence it often feels like whenever in English.

So:

• Kad pada kiša... = When it rains...
• In natural English, this sentence is often best understood as Whenever it rains...

That is because the sentence describes a repeated habit, not just one specific moment.

Yes. Padati kiša / pada kiša is a normal Croatian way to say that it is raining.

Literally, it is closer to rain is falling.

A few important points:

• Croatian does not need a dummy subject like English it
• kiša is the real subject
• pada is the 3rd person singular verb form, agreeing with kiša

So:

• Pada kiša. = It is raining.
• Literally: Rain is falling.

You may also hear Kiša pada, which means the same thing.

This is a very common learner question, because it involves aspect.

• staviti → perfective
• stavljati / stavljam → imperfective

In this sentence, stavim is used because it refers to a single completed action each time it rains: I put the hood on.

So the idea is:

• Kad pada kiša, uvijek stavim...
  = Whenever it rains, I put on...
  = each time, I do that completed action

If you said stavljam, that would sound more like a general ongoing or repeated habit, and it can also be possible in some contexts. But stavim is very natural here because Croatian often uses a perfective present for what happens every time a certain situation occurs.

A simple way to feel the difference:

• stavljam = I am putting / I usually put, focusing more on the activity
• stavim = I put it on, focusing on the finished act

Because it is the direct object of the verb stavim.

The base form is:

• kapuljača = hood

But after staviti here, it goes into the accusative singular:

• stavim kapuljaču = I put on the hood

This is a regular pattern for many feminine nouns ending in -a:

• nominative: žena
• accusative: ženu

So:

• nominative: kapuljača
• accusative: kapuljaču

Because Croatian uses different cases after na depending on whether you mean:

• movement toward a place → accusative
• location in a place → locative

Here there is movement:

• stavim kapuljaču na glavu
  = I put the hood onto my head

So Croatian uses na + accusative:

• glava → glavu

Compare:

• Kapuljača mi je na glavi.
  = The hood is on my head.
  Here it is already there, so this is location, and you get na glavi.

So:

• na glavu = onto the head
• na glavi = on the head

Because with body parts, Croatian very often leaves out the possessive if it is already obvious.

So:

• stavim kapuljaču na glavu naturally means I put the hood on my head

Croatian does this much more often than English. The ownership is understood from the subject and the situation.

You could add svoju for emphasis:

• stavim kapuljaču na svoju glavu

But that sounds heavier and is usually unnecessary here.

Because Croatian often drops subject pronouns when the verb ending already shows who the subject is.

Here:

• stavim = I put on

The ending -im already tells you it is 1st person singular, so ja is not needed.

Croatian only adds ja when there is some special reason, for example:

• emphasis
• contrast
• clarification

So:

• Kad pada kiša, uvijek stavim kapuljaču na glavu. = normal
• Ja uvijek stavim kapuljaču na glavu... = more emphatic, like I always do

Yes, you can. Both are grammatical and mean the same thing.

• Kad pada kiša
• Kad kiša pada

Croatian word order is fairly flexible. The first version is especially common and natural in weather expressions, but the second is also fine.

The difference is more about rhythm and emphasis than basic meaning.

Kapuljača means the hood on a jacket, coat, sweatshirt, and so on.

It is specifically the clothing hood you pull over your head.

Useful note:

• clothing hood = kapuljača
• car hood/bonnet = hauba

So in this sentence, kapuljača can only mean the hood of something you are wearing.

Yes, it is natural and correct. But Croatian also has other natural ways to say something similar.

For example:

• stavim kapuljaču na glavu = I put the hood on my head
• navučem kapuljaču = I pull my hood up / over my head
• stavim kapuljaču = I put on my hood

The sentence you were given is perfectly good, but in everyday speech you may also hear navučem kapuljaču, especially if the idea is physically pulling it up over your head.

Because Kad pada kiša is a subordinate clause placed before the main clause.

So the structure is:

• Kad pada kiša, = subordinate clause
• uvijek stavim kapuljaču na glavu. = main clause

In Croatian, when this kind of time clause comes first, it is normally separated by a comma.

If you reverse the order:

• Uvijek stavim kapuljaču na glavu kad pada kiša.

that is also natural, and the punctuation may be lighter because the subordinate clause comes after the main one.