A erva daninha cresce depressa.

Portuguese often uses the definite article where English would not, especially when talking about something in a general sense.

So A erva daninha cresce depressa can mean something like Weeds grow quickly or The weed grows quickly, depending on context.

Using a here sounds natural in Portuguese for a general statement. Without the article, erva daninha cresce depressa, the sentence would sound less normal in most everyday contexts.

Because erva daninha is a set expression.

• erva = herb, grass, plant
• daninha = harmful, noxious

So literally it is something like harmful plant/herb, but together it means weed.

This is very common in Portuguese: a concept that is one word in English may be expressed with two words in Portuguese.

In Portuguese, adjectives often come after the noun.

So:

• erva daninha = weed
• literally plant harmful

This noun + adjective order is very normal in Portuguese, especially when the adjective is classifying the noun rather than adding a subjective opinion.

In fact, erva daninha is such a common expression that learners should treat it almost like a fixed phrase.

Daninha comes from the idea of causing damage or being harmful.

So erva daninha is a plant considered unwanted because it harms crops, gardens, or cultivated land. In English, the natural translation is simply weed.

Also notice the feminine form:

• masculine: daninho
• feminine: daninha

It is feminine here because erva is a feminine noun.

Cresce is the 3rd person singular of the verb crescer = to grow.

The subject is a erva daninha, which is grammatically singular, so the verb must also be singular:

• a erva daninha cresce = the weed grows
• as ervas daninhas crescem = weeds grow

So cresce matches the singular subject.

Portuguese uses the simple present very often for:

• general truths
• habits
• things that usually happen

So A erva daninha cresce depressa means that weeds grow quickly in general.

If you wanted to say is growing right now, European Portuguese would normally use:

• A erva daninha está a crescer depressa

But in your sentence, the simple present is being used for a general fact, which is completely natural.

Both can mean quickly, but depressa is very common and natural in everyday speech.

• depressa = quickly, fast
• rapidamente = rapidly, quickly

In many contexts they are interchangeable, but depressa often sounds more everyday and conversational.

So:

• A erva daninha cresce depressa = very natural
• A erva daninha cresce rapidamente = also correct, slightly more formal or neutral

Because the sentence needs an adverb, not an adjective.

• rápido / rápida are adjectives: fast, quick
• depressa is an adverb: quickly

Here, the word describes how it grows, so an adverb is needed:

• cresce depressa = grows quickly

You cannot normally say cresce rápida here, because that would be an adjective and would not fit the structure.

Grammatically, it is singular:

• a erva daninha

But it can still express a general idea, much like English sometimes uses a singular noun to talk about a whole class.

Compare:

• A erva daninha cresce depressa. = Weeds grow quickly / The weed grows quickly.
• As ervas daninhas crescem depressa. = Weeds grow quickly.

Both can express a general truth. Portuguese often allows either singular or plural for this kind of statement, depending on style and context.

A rough European Portuguese pronunciation is:

uh AIR-vuh duh-NEE-nyuh KRESH-suh duh-PRESS-uh

A few useful points:

• erva: the first e is open, roughly like eh
• nh in daninha sounds like ny in canyon
• cresce has sc pronounced like ss here
• depressa has stress on pres

This is only an approximation, but it helps an English speaker get closer to the European Portuguese sound.

Yes, absolutely.

That is the plural version:

• As ervas daninhas crescem depressa = Weeds grow quickly

This may feel more natural to an English speaker because English often uses the plural for general statements like this. Portuguese can use either the singular or the plural in such cases, though the exact nuance can vary a little with context and style.