Сегодня я взяла куртку потеплее, потому что утром было пасмурно.

In the past tense, Russian verbs agree with the subject in gender and number.

• я взяла = I took / said by a female speaker
• я взял = I took / said by a male speaker
• мы взяли = we took

So взяла tells you the speaker is female.

Because it is the direct object of the verb взяла.

The verb взять takes the accusative case, and куртка is a feminine noun ending in -а, so in the accusative singular it becomes куртку.

• nominative: куртка
• accusative: куртку

So:

• Я взяла куртку = I took a jacket

Потеплее means a bit warmer or somewhat warmer.

It comes from the comparative теплее = warmer, with the prefix по-, which often softens the meaning and makes it sound more natural in everyday speech:

• теплее = warmer
• потеплее = a little warmer / somewhat warmer

So куртку потеплее means something like:

• a warmer jacket
• a slightly warmer jacket

Both are possible, but they sound a little different.

• куртку потеплее is very natural, conversational Russian
• более тёплую куртку is more formal and bookish

Russian very often uses this pattern:

• noun + comparative

For example:

• чай покрепче = stronger tea
• квартиру побольше = a bigger apartment
• куртку потеплее = a warmer jacket

So this sentence uses a very common spoken pattern.

Because in Russian, when a comparative means a more X one, it often follows the noun.

So instead of building a full adjective phrase, Russian can say:

• куртку потеплее
• literally: a jacket warmer

This is normal and idiomatic. English usually puts the adjective before the noun, but Russian often does not in this kind of structure.

Because Russian describes weather here with an impersonal construction.

• пасмурно is not a normal adjective here
• it is a predicative word meaning cloudy / overcast

So:

• было пасмурно = it was cloudy / overcast

Russian does not use a dummy subject like English it. English says it was cloudy, but Russian simply says было пасмурно.

In impersonal sentences like weather expressions, Russian usually uses the neuter singular past tense form.

That is why you get:

• было холодно = it was cold
• было темно = it was dark
• было пасмурно = it was cloudy

There is no real grammatical subject, so Russian uses this default neuter form.

Утром is the instrumental singular of утро.

In Russian, some time expressions use the instrumental case adverbially:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So утром было пасмурно means it was cloudy in the morning.

Because потому что introduces a subordinate clause meaning because.

Russian normally puts a comma before this conjunction:

• Сегодня я взяла куртку потеплее, потому что утром было пасмурно.

This is standard Russian punctuation.

Yes. Russian word order is fairly flexible.

This sentence begins with Сегодня to set the time frame first:

• Сегодня я взяла куртку потеплее...

But you could also say:

• Я сегодня взяла куртку потеплее...

Both are correct. The difference is mostly one of emphasis and flow, not basic meaning.

Starting with Сегодня gives today a little more prominence.

Взяла is the perfective past tense of взять, and it presents the action as a single completed act.

Here the speaker is talking about one finished choice:

• Today I took/chose a warmer jacket

That is why взяла fits well.

Брала is imperfective and would sound different. It could suggest:

• repeated action
• process
• background description
• a different context

So in this sentence, взяла is the natural choice.

Literally, взяла means took. In context, it often means picked/chose.

So here it suggests something like:

• I took a warmer jacket
• I chose a warmer jacket

It does not specifically mean put on. If you want to say put on, Russian would more likely use надела:

• Сегодня я надела куртку потеплее = Today I put on a warmer jacket

So взяла focuses on selecting or taking the jacket, not necessarily the act of putting it on.