Сегодня миксер сломался, поэтому мне пришлось взбивать всё вручную.

Because it agrees with миксер, which is a masculine singular noun in Russian. In the past tense, Russian verbs change for gender and number:

• masculine: сломался
• feminine: сломалась
• neuter: сломалось
• plural: сломались

So миксер сломался means the mixer broke.

The verb here is сломаться, which means to break down / to get broken. The -ся ending often makes a verb reflexive or changes its meaning in other ways. In this case, it gives the sense that the machine broke or stopped working, rather than someone actively breaking something.

Compare:

• Я сломал миксер. = I broke the mixer.
• Миксер сломался. = The mixer broke / broke down.

So сломался is the natural choice for a machine that stopped working.

Russian word order is flexible, and putting Сегодня first gives the time frame right away: Today, ...

This is very natural in Russian. You could move it:

• Сегодня миксер сломался...
• Миксер сегодня сломался...

Both are possible, but the first version sounds very neutral and natural if you want to start by setting the scene in time.

Поэтому means therefore, so, or that’s why. It connects the cause and the result:

• Сегодня миксер сломался = the cause
• поэтому мне пришлось... = the result

So the logic is:

The mixer broke today, so I had to whisk everything by hand.

Because поэтому connects two clauses, and in Russian this normally requires a comma:

• Сегодня миксер сломался, поэтому мне пришлось...

This is similar to English punctuation in sentences like The mixer broke, so I had to do everything by hand.

Because пришлось comes from прийтись / приходиться, which in this meaning works impersonally and takes the person in the dative case.

So Russian says something like:

• мне пришлось = it was necessary for me / I had to
• тебе пришлось = you had to
• ему пришлось = he had to

That is why you use мне and not я.

Because this is an impersonal construction. There is no normal subject like я controlling the verb. In such cases, Russian often uses the past tense in neuter singular:

• мне пришлось
• нам пришлось
• ей пришлось

The form stays пришлось regardless of who had to do it. What changes is the dative pronoun:

• мне пришлось = I had to
• тебе пришлось = you had to
• им пришлось = they had to

Both can express necessity, but they are not exactly the same.

• мне пришлось = I had to, often because of circumstances, often with a sense that it was unavoidable
• мне нужно было = I needed to / I was supposed to, which can sound less forced

In this sentence, пришлось is very natural because the mixer broke, so the speaker had no choice.

• Мне пришлось взбивать всё вручную. = I had to whisk everything by hand.
• Мне нужно было взбивать всё вручную. = I needed to whisk everything by hand.

The first one better expresses unwanted necessity caused by the situation.

After пришлось, either aspect can be possible, depending on what you want to emphasize.

Here, взбивать is imperfective and focuses on the process or activity itself: the speaker had to spend time whisking everything manually.

• пришлось взбивать = had to do the whisking, had to be whisking

If you used взбить, that would focus more on completing the result:

• пришлось взбить всё вручную = had to whisk everything by hand, with more emphasis on getting it done

So in this sentence, взбивать highlights the labor/process, which fits well with the idea that the mixer broke and the manual work was inconvenient.

Взбивать means to beat, to whisk, or to whip, usually for food mixtures such as eggs, cream, batter, and so on.

It is the imperfective verb. Its perfective partner is взбить.

Examples:

• взбивать яйца = to beat eggs
• взбить сливки = to whip cream

In your sentence, it means the speaker had to whisk or beat the mixture manually.

Всё means everything here. It is the direct object of взбивать, so it is in the accusative case.

For this word, nominative and accusative look the same:

• nominative: всё
• accusative: всё

So even though the form does not change, its role in the sentence is object: the speaker had to whisk everything.

Вручную means by hand or manually. It is an adverb.

• вручную = manually, by hand
• руками = with one’s hands

Both can be similar, but вручную is the more natural choice when contrasting manual work with using a machine or tool.

So here:

• взбивать всё вручную = whisk everything by hand, manually

This fits perfectly because the mixer stopped working.

Yes. Both are natural, but they emphasize slightly different things.

• Миксер сломался. = The mixer broke.
• У меня сломался миксер. = My mixer broke / I had my mixer break.

Adding у меня makes it more personal and gives the sense my mixer or the mixer I have. The sentence in your example is a bit more neutral and direct.

It can be rearranged, because Russian word order is flexible. The original sentence is very natural and neutral:

• Сегодня миксер сломался, поэтому мне пришлось взбивать всё вручную.

Other versions are possible, for example:

• Миксер сегодня сломался, поэтому мне пришлось взбивать всё вручную.
• Сегодня сломался миксер, поэтому мне пришлось взбивать всё вручную.

These are all understandable, but they may shift emphasis slightly. The original version sounds smooth and standard.

It describes a one-time event.

You can see that from several clues:

• Сегодня = today
• сломался = perfective past, a completed event: the mixer broke
• пришлось = had to, because of that particular situation

So the sentence means that today, because the mixer broke, the speaker had to whisk everything by hand on that occasion.