Октябрь был дождливым, поэтому мой чёрный зонт всегда был со мной.

Because after быть in the past or future, Russian often uses the instrumental case for a predicate noun or adjective:

• Октябрь был дождливым = October was rainy

This is a very common pattern:

• Он был уставшим = He was tired
• Она была врачом = She was a doctor

You may sometimes also see the nominative in similar sentences, but the instrumental is very standard here.

It is the instrumental singular masculine form of the adjective дождливый (rainy).

Why masculine singular?

Because it describes Октябрь, and октябрь is:

• masculine
• singular
• the subject of the sentence

In predicate position after был, the adjective changes to instrumental:

• дождливый → дождливым

Because мой чёрный зонт is the subject of the second clause:

• мой чёрный зонт всегда был со мной

The subject is usually in the nominative case. All three words agree with each other:

• мой = masculine singular nominative
• чёрный = masculine singular nominative
• зонт = masculine singular nominative

They all match because зонт is masculine singular.

Russian sometimes uses со instead of с for easier pronunciation.

Since мной begins with a difficult consonant combination after с, Russian prefers:

• со мной = with me

Compare:

• с братом = with my brother
• со мной = with me
• со всеми = with everyone

So this is mainly a pronunciation/euphony rule.

Мной is the instrumental case of я (I / me).

The preposition с / со usually requires the instrumental when it means with:

• со мной = with me
• с тобой = with you
• с ним = with him

So in зонт всегда был со мной, the phrase literally means the umbrella was always with me.

Because the sentence has two clauses, and each clause has its own verb:

1. Октябрь был дождливым
2. мой чёрный зонт всегда был со мной

In English, you also repeat was:

• October was rainy, so my black umbrella was always with me.

Russian does the same here.

Поэтому means therefore, so, or that’s why.

It introduces a result:

• Октябрь был дождливым, поэтому...
• October was rainy, so...

It commonly appears after a comma when connecting two parts of a sentence.

Not completely. Russian word order is more flexible than English.

The neutral order here is:

• Октябрь был дождливым, поэтому мой чёрный зонт всегда был со мной.

But всегда could move:

• ...поэтому мой чёрный зонт был всегда со мной
• ...поэтому всегда мой чёрный зонт был со мной

These are possible, but the original sounds natural and neutral.

Russian word order often changes to shift emphasis rather than basic meaning.

Russian often expresses this idea with being with someone:

• зонт был со мной = literally the umbrella was with me

This naturally means:

• I had my umbrella with me
• my umbrella was always with me

So the Russian structure is different from English, but very normal.

Yes. In many Russian texts, ё is often written as е, so you may see:

• чёрный
• черный

Both represent the same word: black.

However, ё shows the correct pronunciation more clearly, and in learning materials it is very helpful to keep it.

Months are normally lowercase in Russian:

• январь
• февраль
• октябрь

Here it is capitalized because it is the first word of the sentence. If it were in the middle, it would usually be:

• октябрь был дождливым

So this is sentence capitalization, not a special rule for months.

No, not in the past tense.

In the present tense, Russian usually omits to be:

• Октябрь дождливый = October is rainy
• Зонт со мной = The umbrella is with me

But in the past tense, you must use the past form of быть:

• Октябрь был дождливым
• Зонт был со мной

So был is required here.

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

• Октябрь был because октябрь is masculine singular
• зонт был because зонт is also masculine singular

Compare:

• погода была = the weather was
• окно было = the window was
• дни были = the days were

So the form был matches both masculine subjects in this sentence.