Quando a sopa começa a ferver, eu mexo devagar e fecho a tampa da panela.

Because começar normally takes the preposition a before another verb in the infinitive.

• começar a ferver = to start boiling
• começar a cozinhar = to start cooking
• começar a chover = to start raining

So in this sentence, a ferver is the natural pattern after começa.

Ferver means to boil or to be boiling.

In the sentence:

• a sopa começa a ferver = the soup starts to boil

It refers to the liquid reaching the boiling point. You will often see it in cooking contexts.

Portuguese often uses the definite article more than English does.

• a sopa = the soup

Even when English might simply say soup in a general cooking instruction, Portuguese commonly says a sopa if the speaker has a specific soup in mind. In this sentence, it sounds natural because we are talking about the soup being cooked.

Yes, it can be omitted.

Portuguese verbs usually show who the subject is, so mexo already tells you the subject is I.

• eu mexo devagar = I stir slowly
• mexo devagar = also I stir slowly

Including eu can add clarity, emphasis, or rhythm, but it is not required here.

Here, mexo means I stir.

The verb mexer has several meanings depending on context, such as:

• to stir a liquid or food
• to move
• to touch / mess with something

In cooking, mexer very often means to stir:

• mexer a sopa = to stir the soup
• mexer o molho = to stir the sauce

So context is very important with this verb.

Both are possible.

• eu mexo devagar = I stir slowly
• eu mexo a sopa devagar = I stir the soup slowly

In this sentence, the object is understood from context: we are already talking about the soup. Portuguese often leaves out an object when it is obvious.

Both can mean slowly, but devagar is more common and more natural in everyday speech.

• mexo devagar = very natural
• mexo lentamente = correct, but a bit more formal or descriptive

In cooking instructions, devagar sounds very normal and idiomatic.

Because a tampa is the direct object of fechar.

• fechar a tampa = to close the lid
• fechar a porta = to close the door

You use com when it means with, but here the lid is not a tool; it is the thing being closed.

So:

• fecho a tampa da panela = I close the pot’s lid / the lid of the pot

Da is a contraction of de + a.

• de = of
• a panela = the pot
• de + a = da

So:

• a tampa da panela = the lid of the pot

This contraction is required in normal Portuguese when de comes before the feminine singular article a.

Portuguese usually expresses possession with de rather than with an English-style apostrophe structure.

So the normal pattern is:

• a tampa da panela = the lid of the pot
• a porta da casa = the door of the house
• o nome do professor = the teacher’s name

This is one of the most common possession patterns in Portuguese.

The whole sentence is in the present tense:

• começa
• mexo
• fecho

Even though it is in the present, it can describe:

1. a habitual action
2. a step-by-step instruction
3. something that generally happens during cooking

Portuguese often uses the present tense where English might also use the present in recipes or instructions.

Because the sentence describes a general or repeated situation:

• Quando a sopa começa a ferver... = When the soup starts to boil...

Portuguese often uses the present after quando for actions that happen regularly or as part of a process. This is very normal in instructions, routines, and general truths.

Yes. In Portuguese, the first person singular can be used in a personal description of a process, such as explaining how someone cooks.

It sounds like the speaker is saying what they do:

• When the soup starts to boil, I stir slowly and close the pot lid.

If it were a direct instruction to you, Portuguese might say:

• Quando a sopa começar a ferver, mexa devagar e feche a tampa da panela.

So your original sentence sounds more like a personal routine or description.

In standard Brazilian Portuguese:

• mexo sounds roughly like MEH-shoo
• fecho sounds roughly like FEH-shoo

The x here usually has a sh sound.

Also:

• Quando ≈ KWAN-doo
• sopa ≈ SOH-pah
• ferver ≈ fehr-VEHR
• panela ≈ pah-NEH-lah

Exact pronunciation varies by region, but these approximations are useful for a learner.

Panela usually means a pot or a cooking pan, especially a deeper one used on the stove.

Depending on context, English may translate it as:

• pot
• pan
• saucepan

In this sentence, pot is probably the best choice, because soup is typically cooked in a pot.

Yes, in many contexts tampar a panela is a very natural alternative.

• tampar a panela = to put the lid on / to cover the pot
• fechar a tampa da panela = to close the pot lid

Both are understandable, but tampar is especially common when you mean to cover something with a lid.

So a very natural cooking version would be:

• Quando a sopa começa a ferver, eu mexo devagar e tampo a panela.

Because articles are used very regularly in Portuguese noun phrases.

Here you have:

• a tampa = the lid
• da panela = of the pot

Both nouns can take their own article, and that is completely normal in Portuguese. English often uses fewer articles, so this can feel repetitive to English speakers, but it is standard Portuguese structure.