Капуста варится быстрее, чем свёкла, поэтому свёклу я кладу в кастрюлю раньше.

Because вариться is the reflexive/intransitive form of варить.

• варить = to boil/cook something
  
  • Я варю суп. = I am cooking soup.
• вариться = to boil/cook, to be cooking
  
  • Суп варится. = The soup is cooking.

So Капуста варится быстрее means Cabbage cooks faster or Cabbage boils faster.

They are the subjects of the comparison.

• Капуста варится быстрее = Cabbage cooks faster
• чем свёкла = than beetroot

Both капуста and свёкла are being compared as the things that cook, so they appear in the nominative.

Because in the second clause it is no longer the subject — it is the direct object of кладу.

• свёкла = nominative
• свёклу = accusative

So:

• свёкла варится = beetroot cooks
• я кладу свёклу = I put beetroot

That is why the form changes.

Чем is the normal word used in Russian comparisons after a comparative form, like быстрее.

• быстрее, чем... = faster than...
• раньше, чем... = earlier than...

So Капуста варится быстрее, чем свёкла means Cabbage cooks faster than beetroot.

It is the comparative form of быстро / быстрый and means faster / more quickly.

In this sentence it functions adverbially, describing how the cabbage cooks:

• быстро = quickly
• быстрее = more quickly / faster

So варится быстрее = cooks faster.

They express two different ideas:

• быстрее = faster, more quickly
  This refers to the speed of cooking.
• раньше = earlier
  This refers to time order.

So:

• Капуста варится быстрее = Cabbage cooks faster.
• Свёклу я кладу ... раньше = I put the beetroot in earlier.

Кладу is the 1st person singular present tense of класть: I put / I am putting.

This verb is imperfective, so here it suggests a usual or repeated action, not a one-time completed action.

• класть = to put, to place
• я кладу = I put

In context, it means something like that’s why I put the beetroot into the pot earlier.

Yes, but the meaning would shift slightly.

• кладу = imperfective; habitual, general, or process-focused
• положу = perfective; one completed future action

So:

• поэтому свёклу я кладу в кастрюлю раньше = that’s why I put beetroot in earlier / that’s what I do
• поэтому свёклу я положу в кастрюлю раньше = therefore I will put the beetroot into the pot earlier

In this sentence, кладу fits well because it sounds like a general cooking practice.

Because в can take either the accusative or the prepositional case depending on meaning.

• в + accusative = motion into somewhere
• в + prepositional = location in/inside somewhere

Here the beetroot is being put into the pot, so Russian uses the accusative:

• в кастрюлю = into the pot

Compare:

• Я кладу свёклу в кастрюлю. = I put beetroot into the pot.
• Свёкла уже в кастрюле. = The beetroot is already in the pot.

Russian word order is flexible. Putting свёклу first gives it a bit more emphasis or topic focus.

Both are possible:

• Я кладу свёклу в кастрюлю раньше.
• Свёклу я кладу в кастрюлю раньше.

The version in the sentence sounds a little more like As for the beetroot, I put it in earlier. It helps connect back to the comparison with свёкла in the first clause.

Yes, Russian often omits subject pronouns when the verb form already makes the subject clear.

So кладу already tells you it is I.

However, я is included here for emphasis or clarity. It can sound a bit more contrastive or personal:

• поэтому свёклу я кладу... = therefore I put the beetroot in earlier

This is very natural in Russian.

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

It links the cause and the result:

• Cause: Капуста варится быстрее, чем свёкла
• Result: поэтому свёклу я кладу в кастрюлю раньше

So the logic is: Cabbage cooks faster than beetroot, so I put the beetroot into the pot earlier.

Strictly speaking, the correct form is свёкла with ё.

In everyday Russian, however, ё is often written as е, so you may also see свекла in texts. But it is still pronounced with ё:

• свёкла = pronounced roughly svyok-la

For a learner, it is useful to remember the dictionary form with ё.

Grammatically it is singular feminine, but in meaning it behaves like a mass noun, similar to English cabbage.

So here it means cabbage as an ingredient or vegetable in general, not necessarily one whole cabbage head.

The same is true for свёкла: it means beetroot as a food item or ingredient.

It can be understood mainly as a general cooking fact plus a usual practice.

• Капуста варится быстрее, чем свёкла sounds like a general truth.
• поэтому свёклу я кладу в кастрюлю раньше sounds like a habitual action: that’s why I put beetroot in earlier.

So the whole sentence naturally reads like an explanation of someone’s normal method when cooking.