A beterraba deixa as mãos vermelhas.

In Portuguese, it is very common to use the definite article with a noun when talking about something in a general sense.

So A beterraba deixa as mãos vermelhas can mean something like Beetroot makes your hands red or Beetroot leaves the hands red.

This is more natural in Portuguese than dropping the article.
Compare:

• A beterraba é saudável. = Beetroot is healthy.
• O vinho é caro. = Wine is expensive.

English often uses no article in these general statements, but Portuguese usually does use one.

Here, deixa comes from the verb deixar, which very often means to leave.

In this sentence, it means to leave/make something in a certain state:

• deixar as mãos vermelhas = to leave the hands red

So the structure is:

• deixar + object + adjective

Examples:

• O sol deixa a pele escura. = The sun makes/leaves the skin dark.
• A tinta deixou a camisa azul. = The paint left the shirt blue.

So in this sentence, deixa is not about allowing something. It is about the result or effect.

Because the subject is a beterraba, which is singular.

• a beterraba = singular
• therefore deixa = 3rd person singular

If the subject were plural, the verb would also be plural:

• As beterrabas deixam as mãos vermelhas.

Portuguese verbs must agree with the subject in number and person.

Portuguese often uses the definite article with body parts, especially when the owner is understood from the context.

So:

• as mãos = the hands

In English, you often say your hands in this kind of sentence. In Portuguese, it is very natural to say as mãos instead.

For example:

• Lava as mãos. = Wash your hands.
• Doe-me a cabeça. = My head hurts.
• Ela fechou os olhos. = She closed her eyes.

The article is usually preferred when it is obvious whose body part is meant.

Because minhas is not necessary here.

In Portuguese, if the owner is clear from the situation, speakers often prefer the article alone:

• as mãos instead of as minhas mãos

Using as minhas mãos is possible, but it sounds more specific or emphatic. It may suggest contrast, for example:

• A beterraba deixa as minhas mãos vermelhas, mas não as tuas.

In a general statement, as mãos is the most natural choice.

Because vermelhas agrees with mãos.

• mão is a feminine noun
• mãos is plural
• so the adjective must also be feminine plural

Agreement:

• a mão vermelha
• as mãos vermelhas

This is a basic rule in Portuguese: adjectives usually agree in gender and number with the noun they describe.

Yes. Mão is an irregular case.

Although many Portuguese nouns ending in -o are masculine, mão is feminine:

• a mão
• as mãos

So you say:

• a mão esquerda
• as mãos vermelhas

This is something you simply need to memorize, because the ending does not follow the most common pattern.

The pattern is:

• subject + deixar + object + adjective

Here:

• A beterraba = subject
• deixa = verb
• as mãos = direct object
• vermelhas = adjective describing the object

So vermelhas is not describing beterraba. It describes mãos.

A helpful way to think about it is:

• The beetroot leaves the hands red.

This pattern is very common in Portuguese:

• Isso deixa-me triste. = That makes me sad.
• O café deixa-o nervoso. = Coffee makes him nervous.

Yes, absolutely.

• A beterraba mancha as mãos. = Beetroot stains the hands.

This is a very natural alternative.

The difference is small:

• deixa as mãos vermelhas focuses on the resulting state: the hands end up red
• mancha as mãos focuses more directly on staining

Both are good Portuguese. The original sentence is slightly more descriptive.

Yes, but the structure changes.

You could say:

• As mãos ficam vermelhas com a beterraba.
  = The hands get red with beetroot.

This version makes as mãos the subject.

Compare:

• A beterraba deixa as mãos vermelhas.
• As mãos ficam vermelhas com a beterraba.

Both are natural, but they organise the sentence differently:

• the first focuses on beterraba as the cause
• the second focuses on mãos and the change they undergo

Yes. Beterraba is the standard word in European Portuguese for beetroot or beet.

So this sentence sounds normal in Portugal:

• A beterraba deixa as mãos vermelhas.

Depending on context, English might translate it as beetroot rather than beet, since beterraba often refers to the vegetable as a whole.

A rough pronunciation guide is:

• A beterraba deixa as mãos vermelhas
• roughly: uh beh-teh-RAH-buh DAY-shuh az MOWNZH vehr-MEH-lyush

A few points for European Portuguese:

• deixa sounds like DAY-shuh
• as mãos links together in speech, and s in as sounds like z before the next word
• mãos has a nasal sound
• the ending -lhas in vermelhas is softer than it may look to an English speaker

This is only an approximation, but it can help you recognise the sentence when you hear it.