בערב אני מרתיחה מים בסיר גדול למרק.

בערב means in the evening or at evening.

It is made from:

• ב־ = in / at
• ערב = evening

In normal unpointed Hebrew spelling, בערב can also reflect ב + הערב, so in practice learners often understand it as in the evening.

Putting it at the start gives a natural time-first structure:

• בערב אני מרתיחה מים... = In the evening, I boil water...

That is very common in Hebrew.

In this sentence, אני is helpful and basically needed to mean I clearly.

That is because מרתיחה is a present-tense form, and Hebrew present-tense forms do not show person clearly.
מרתיחה only tells you:

• feminine
• singular
• present

So by itself, מרתיחה could mean:

• I am boiling
• you are boiling (to one female)
• she is boiling

Because of that, Hebrew usually includes the pronoun in cases like this:

• אני מרתיחה = I am boiling

Because the speaker is feminine singular.

In Hebrew present tense, verbs agree with gender and number:

• מרתיח = masculine singular
• מרתיחה = feminine singular
• מרתיחים = masculine plural / mixed plural
• מרתיחות = feminine plural

So:

• אני מרתיחה = said by a woman
• אני מרתיח = said by a man

מרתיחה comes from the verb להרתיח, which means to boil, to bring to a boil, or to make something boil.

This is different from the intransitive idea to be boiling.

Compare:

• אני מרתיחה מים = I boil / I am boiling water
• המים רותחים = the water is boiling

So להרתיח is the verb you use when someone causes the water to boil.

Yes, in English water is usually singular/uncountable, but Hebrew מים is the normal word for water, and it has a plural form.

This is just how the noun works in Hebrew. Some mass nouns behave differently across languages, and מים is one of those cases.

So even though it looks plural, it simply means water.

A learner should just memorize:

• מים = water

Because את is used before a definite direct object, and מים here is indefinite.

So:

• אני מרתיחה מים = I boil water
• אני מרתיחה את המים = I boil the water

In the sentence you gave, the meaning is general, not the water, so את is not used.

Because in Hebrew, adjectives normally come after the noun, and they must agree with it.

Here:

• סיר = pot
• גדול = big

So:

• סיר גדול = a big pot

The adjective must match the noun in:

• gender
• number
• definiteness

Since סיר is masculine singular, the adjective is also masculine singular:

• גדול

Examples:

• סיר גדול = a big pot
• סיר קטן = a small pot
• הסיר הגדול = the big pot

Literally, it means in a big pot.

It breaks down as:

• ב־ = in
• סיר = pot
• גדול = big

So:

• בסיר גדול = in a big pot

If it were in the big pot, Hebrew would usually say:

• בסיר הגדול

Here it most naturally means for soup or for making soup.

Breakdown:

• ל־ = often for / to
• מרק = soup

So:

• למרק = for soup

However, this is a very good learner question, because in normal Hebrew spelling למרק can also represent ל + המרק, which would mean to the soup or for the soup. Hebrew often leaves out vowel markings, so context decides.

In your sentence, the natural interpretation is:

• I boil water in a big pot for soup

Grammatically, it is present tense, but Hebrew present tense can often express either:

1. what someone is doing now, or
2. a regular habit

So this sentence could mean:

• I am boiling water in a big pot for soup this evening
• or In the evening, I boil water in a big pot for soup

Context tells you which one is meant.

Without more context, many learners would read it as a habitual or general statement:

• In the evening, I boil water in a big pot for soup

Hebrew word order is fairly flexible, though some orders sound more natural than others.

Your sentence:

• בערב אני מרתיחה מים בסיר גדול למרק

This is very natural because it starts with the time expression and then moves to the subject and action.

You could also hear other arrangements, depending on emphasis, such as:

• אני מרתיחה מים בערב בסיר גדול למרק
• מים אני מרתיחה בערב בסיר גדול למרק
  This one sounds more marked and puts emphasis on water

So the sentence you gave is normal and smooth, but Hebrew does allow reordering for focus or style.