Сегодня утром было пасмурнее, чем вчера, поэтому я взял зонтик.

Because this is an impersonal weather expression.

In Russian, weather and general conditions are often said without a real subject:

• было холодно = it was cold
• было темно = it was dark
• было пасмурнее = it was more overcast

In the past tense, these impersonal sentences usually use neuter singular:

• было

So было пасмурнее literally works like it was more overcast, even though Russian does not use a separate word for it here.

Yes. Сегодня утром is a very natural way to say this morning.

It is made of:

• сегодня = today
• утром = in the morning

So together:

• сегодня утром = this morning / today in the morning

Russian often uses time words this way:

• сегодня днём = this afternoon / during the day today
• сегодня вечером = this evening
• сегодня ночью = tonight / during the night today

Because утром is the instrumental case, and in Russian some time expressions use the instrumental to mean during or in a part of the day.

So:

• утро = morning
• утром = in the morning

This is very common:

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

So сегодня утром is literally something like today, in the morning.

In this sentence, it functions as a comparative predicate, meaning more overcast.

It comes from пасмурный / пасмурно:

• пасмурный = overcast, gloomy
• пасмурно = it is overcast / gloomy
• пасмурнее = more overcast

With weather, Russian often uses the predicate form rather than a full adjective with a noun:

• Сегодня пасмурно. = Today it is overcast.
• Сегодня было пасмурнее, чем вчера. = This morning it was more overcast than yesterday.

So here пасмурнее is best understood as more overcast, not as an adjective describing a noun.

Russian usually makes comparisons with:

• comparative form + чем = more/less ... than ...

So here:

• пасмурнее = more overcast
• чем = than
• вчера = yesterday

Together:

• пасмурнее, чем вчера = more overcast than yesterday

Other examples:

• лучше, чем раньше = better than before
• быстрее, чем я думал = faster than I thought
• холоднее, чем вчера = colder than yesterday

Because вчера is an adverb, not a noun.

Adverbs usually do not decline for case. So after чем, you can simply say:

• чем вчера = than yesterday
• чем раньше = than before
• чем обычно = than usual

If the second part of the comparison is a noun or pronoun, then case issues can appear in other structures, but with an adverb like вчера, nothing changes.

Because поэтому introduces the result of the first clause, and the sentence contains two clauses:

1. Сегодня утром было пасмурнее, чем вчера
2. поэтому я взял зонтик

Russian normally separates such clauses with a comma.

Here поэтому means:

• therefore
• so
• that’s why

So the structure is:

• It was more overcast this morning than yesterday, so I took an umbrella.

No, they are different.

• поэтому = therefore / so / that’s why
  It introduces a result.
• потому что = because
  It introduces a reason.

So:

• Сегодня утром было пасмурнее, чем вчера, поэтому я взял зонтик.
  = It was more overcast this morning than yesterday, so I took an umbrella.

Compare:

• Я взял зонтик, потому что сегодня утром было пасмурнее, чем вчера.
  = I took an umbrella because it was more overcast this morning than yesterday.

Both are correct, but the structure and emphasis are different.

Взял is the past tense masculine singular form of взять.

It tells you three things:

• past tense: the action happened in the past
• masculine: the speaker is male (or a male subject is being talked about)
• perfective aspect: the action is viewed as completed

So я взял зонтик means:

• I took an umbrella
• I grabbed an umbrella
• I took one with me

If the speaker were female, it would be:

• я взяла зонтик

If you used the imperfective брал, the nuance would be different and less natural here for a single completed decision.

Because зонтик is the direct object of взял, and it is inanimate masculine.

For inanimate masculine nouns, the accusative singular is the same as the nominative singular:

• nominative: зонтик
• accusative: зонтик

Compare with an animate masculine noun:

• я вижу брата
  Here брат becomes брата in the accusative because it is animate.

So:

• я взял зонтик is exactly what you expect grammatically.

Yes, often it can.

Russian frequently drops subject pronouns when the meaning is clear from context and the verb form.

So in conversation, you might hear:

• Сегодня утром было пасмурнее, чем вчера, поэтому взял зонтик.

That can sound natural if it is already obvious who did it.

However, я is also perfectly normal to include:

• for clarity
• when starting a new sentence or idea
• for emphasis or contrast

So the version with я is clear, standard, and natural.

Both зонт and зонтик mean umbrella.

• зонт = umbrella
• зонтик = literally a diminutive form, but very often just a normal everyday word for umbrella

In modern speech, зонтик is extremely common and does not always sound especially cute or childish. It can simply mean umbrella in an ordinary way.

So in this sentence, зонтик is completely natural.

Yes. Russian word order is fairly flexible, though some versions sound more natural than others depending on emphasis.

The given sentence is very natural:

• Сегодня утром было пасмурнее, чем вчера, поэтому я взял зонтик.

You could also say:

• Я взял зонтик, потому что сегодня утром было пасмурнее, чем вчера.
• Поэтому я взял зонтик at the start of a second clause is also normal after the first idea.

The original order works well because it presents:

1. the situation
2. the consequence

That is a very common and natural flow in Russian.