Tulpina este verde.

Because tulpina is the definite form of the noun.

• tulpină = a stem
• tulpina = the stem

In Romanian, the definite article is usually attached to the end of the noun instead of being a separate word like the in English.

So:

• o tulpină = a stem
• tulpina = the stem

The dictionary form is tulpină.

This is the singular indefinite form, the form you would normally look up in a dictionary.
The sentence uses tulpina because it means the stem, not just a stem.

Tulpină is a feminine noun.

You can tell from forms like:

• o tulpină = a stem
• două tulpini = two stems

That feminine gender matters for things like articles and adjective agreement in some situations.

Este is the verb to be, third person singular:

• eu sunt = I am
• tu ești = you are
• el/ea este = he/she/it is

So Tulpina este verde literally has the same structure as English:

• The stem
  • is
    • green

Yes. E is a very common short form of este.

So both are correct:

• Tulpina este verde — a bit more full/formal/neutral
• Tulpina e verde — very common in everyday speech

Both mean the same thing.

Because verde is an adjective whose singular masculine and feminine forms are the same.

So you get:

• un perete verde = a green wall
• o tulpină verde = a green stem

But in the plural, it changes:

• pereți verzi = green walls
• tulpini verzi = green stems

So in this sentence, verde stays the same even though tulpina is feminine singular.

Usually, adjectives like verde come after the noun:

• tulpina verde = the green stem

In your sentence, though, verde comes after este because it is part of the predicate:

• Tulpina este verde = The stem is green

So:

• tulpina verde = the green stem
• tulpina este verde = the stem is green

This is the most normal and neutral word order.

Romanian does allow some flexibility, but Tulpina este verde is the standard way to say it.
You might see different orders for emphasis in special contexts, but as a learner, this is the safest pattern to use:

subject + form of to be + adjective

A simple approximate pronunciation is:

tool-PEE-na ES-te VER-de

A few helpful notes:

• u is like oo in food, but shorter
• i is like ee in see
• e is like e in met
• v and d are pronounced much like in English
• Stress is commonly heard on -pi- in tulpina: tul-PI-na

Normally, no. In standard Romanian, you usually keep the verb to be in sentences like this.

So the normal sentence is:

• Tulpina este verde or
• Tulpina e verde

Leaving it out completely would generally sound incorrect in standard Romanian.

The plural would be:

Tulpinile sunt verzi = The stems are green

Here is what changes:

• tulpina → tulpinile = the stem → the stems
• este → sunt = is → are
• verde → verzi = green → green (plural form)

This is a useful pattern to compare with the singular sentence.

Most commonly, tulpină refers to a stem or stalk, especially in plants.
Depending on context, Romanian can also use it in more technical or figurative ways, but for a beginner, it is best to understand it first as the plant-related stem.

So in a basic sentence like Tulpina este verde, the most natural interpretation is the stem of a plant.