El tallo está verde.

Spanish usually uses estar for a condition or state, and ser for identity, origin, time, and more permanent/classifying descriptions.

In El tallo está verde, verde is being treated as the stem’s current condition: it is green right now.

This is very common with colours when they describe a changing state, especially in nature or food:

• La fruta está verde. = The fruit is unripe / still green.
• Las hojas están verdes. = The leaves are green.

If you used es verde, it would sound more like green is an inherent or defining characteristic.

El is the masculine singular definite article, meaning the.

Spanish uses articles more often than English does. In a normal sentence, a singular count noun like tallo usually needs an article or another determiner:

• el tallo = the stem
• un tallo = a stem
• este tallo = this stem

Just saying Tallo está verde is not grammatical in standard Spanish.

Because tallo is a masculine noun. In Spanish, nouns have grammatical gender, and tallo takes masculine forms:

• el tallo
• un tallo
• este tallo

This gender is grammatical, not biological.

It does agree, but verde is one of those adjectives that has the same form for masculine and feminine in the singular.

So you get:

• el tallo verde
• la hoja verde

The difference appears more clearly in the plural:

• el tallo está verde
• los tallos están verdes
• la hoja está verde
• las hojas están verdes

So verde agrees in number, but not in masculine/feminine form.

In this sentence, verde is not directly placed after the noun inside a noun phrase; it comes after the verb está because it is part of the predicate:

• El tallo está verde.

That structure is very normal in Spanish, just as in English:

• The stem is green.

If you use the adjective directly with the noun, it usually comes after the noun in Spanish:

• el tallo verde = the green stem

So Spanish generally puts descriptive adjectives after the noun, though there are exceptions.

The accent mark shows the correct stress: es-TÁ.

It also helps distinguish forms in writing. Here, está is the he/she/it is or you are form of estar.

Examples:

• El tallo está verde.
• La flor está abierta.

Without the accent, esta is a different word, usually this:

• esta planta = this plant

So the accent is important.

A good approximate pronunciation in Spain is:

TAH-yo

A few details:

• ta- sounds like ta in father, but shorter.
• ll in most of Spain is pronounced like the y in yes.
• The stress is on the first syllable: TA-llo.

In some parts of Spain, ll may sound a bit different, but y is a good learner-friendly approximation.

A simple approximation is:

BER-deh

Helpful notes:

• The v in Spanish is pronounced very similarly to b.
• The r in verde is a light tap, not the strong English r.
• The stress is on the first syllable: VER-de.

So a natural learner pronunciation would be close to BER-deh, with a light tapped r.

It can be just a colour, but in many contexts verde can also suggest that something is not mature yet.

For plants, fruit, and similar things, estar verde often implies still unripe, not ready, or not fully mature.

So depending on context, El tallo está verde may simply describe colour, or it may highlight that the stem is still fresh/young.

Yes, grammatically you can, but it changes the meaning.

• El tallo está verde. = The stem is green.
  This usually refers to a specific stem already known in the context.
• Un tallo está verde. = A stem is green.
  This introduces one stem that is not yet identified to the listener.

So both are possible, but el and un are not interchangeable.

Está is the third person singular form of estar, and it can mean:

• he is
• she is
• it is
• you are (formal singular: usted)

Here it matches el tallo, which is treated as it:

• el tallo está

Usually not here, because el tallo carries the main information about what is green.

Spanish often omits subject pronouns like yo, tú, él, etc., because the verb already shows the person. But nouns are different: if you remove el tallo, you lose the subject entirely.

So:

• El tallo está verde. = complete sentence
• Está verde. = possible only if the context already makes it clear what it is

On its own, Está verde means It is green / It is unripe, but the listener has to know what it refers to.

Yes, tallo is the normal botanical word for a plant’s stem or stalk.

Depending on context, Spanish may use other words too, but tallo is the standard general term for the stem of a plant.

So in a plant-related sentence, El tallo está verde is perfectly natural.