Сегодня я надела сапоги, потому что на улице мокро и холодно.

Because the verb is in the past tense, and in Russian the past tense agrees with the subject in gender when the subject is singular.

• я надела = I put on / wore (said by a woman)
• я надел = the same thing, but said by a man
• мы надели = we put on / wore

So надела tells you the speaker is female.

In standard Russian, надеть means to put on a piece of clothing, while одеть means to dress a person or animal.

So:

• надеть сапоги = to put on boots
• одеть ребёнка = to dress a child

That is why надела сапоги is the correct choice here.

A famous mnemonic is:

• надеть одежду
• одеть Надежду

It is a bit playful, but learners often find it helpful.

Because сапоги is the direct object of надела, and надеть takes the accusative case: put on what? → boots.

For inanimate plural nouns, the accusative plural is often the same as the nominative plural.

So:

• nominative: сапоги
• accusative: сапоги

That is why the form does not visibly change here.

Here it means boots in the normal sense: a pair of boots.

Russian, like English, usually uses the plural for this kind of item:

• сапог = one boot
• сапоги = boots / a pair of boots

So in this sentence, the speaker means she put on boots as footwear.

Because Russian does not need a dummy subject like English it in weather and condition expressions.

English says:

• it is cold
• it is wet

Russian simply says:

• холодно
• мокро

So на улице мокро и холодно literally looks like outside wet and cold, but naturally it means it is wet and cold outside.

Because мокро and холодно are used here as impersonal predicative words describing the general conditions outside.

So:

• на улице холодно = it is cold outside
• на улице мокро = it is wet outside

These words do not agree with a noun here.

If you used adjectives, you would be describing a specific noun instead:

• улица мокрая = the street is wet

That is a different structure. In your sentence, the idea is the overall outdoor condition, not a grammatical subject like улица.

Because Russian normally uses на улице to mean outside, out on the street, or in the street area.

It is just the standard idiomatic expression.

• на улице холодно = it is cold outside
• на улице дождь = it is raining outside / there is rain outside

So even though English learners may expect something like in the street, Russian uses на here.

Because after на when it means location, Russian uses the prepositional case.

The base form is:

• улица = street

After на for location:

• на улице = on the street / outside

So улице is the prepositional singular form of улица.

Russian word order is fairly flexible, and time expressions often come early to set the scene.

So Сегодня я надела сапоги... sounds very natural and means something like:

• As for today, I put on boots...
• Today, I put on boots...

You could also say:

• Я сегодня надела сапоги...

That is also correct. The difference is mainly one of focus and information flow, not basic meaning.

Because потому что introduces a subordinate clause meaning because.

Russian normally places a comma before it:

• Я надела сапоги, потому что на улице холодно.

This is standard punctuation.

Yes. Потому что is the normal, very common way to say because in everyday Russian.

In this sentence:

• Сегодня я надела сапоги, потому что на улице мокро и холодно.
• Today I put on / wore boots because it is wet and cold outside.

So the first clause gives the action, and потому что introduces the reason.

The verb надеть is perfective, and here that makes sense because the sentence refers to a completed action: the speaker put the boots on.

Its imperfective partner is надевать.

Compare:

• я надела сапоги = I put on the boots / completed action
• я надевала сапоги = I was putting on the boots or I used to put on boots, depending on context

Since the sentence is about a concrete choice made today, the perfective надела is the natural form.

Literally, надела means put on. So grammatically, the action is the act of putting the boots on.

But in natural English, depending on context, the sentence may be translated as:

• I put on boots today...
• I wore boots today...

If the point is the speaker’s choice of footwear for the day, English often prefers wore. If the point is the completed action of dressing, put on is closer to the Russian verb.

A useful stress guide is:

• сегОдня
• я надЕла сапогИ
• потомУ что
• на Улице
• мОкро
• холОдно

A rough pronunciation guide:

• sye-VOD-nya ya na-DYE-la sa-pa-GEE, pa-ta-MOO shto na OO-li-tse MOK-ra i kha-LOD-na

The exact sounds are not identical to English, of course, but the stress pattern above is very important.