Lesa galbenă este nouă.

Because Romanian usually puts the definite article at the end of the noun instead of using a separate word like the.

• lesă = a leash
• lesa = the leash

So -a here is the attached definite article. This is very common in Romanian:

• fată = girl
• fata = the girl

They are agreeing with lesa, which is a feminine singular noun.

In Romanian, adjectives must agree with the noun in:

• gender
• number
• sometimes also case

Here:

• lesa = feminine singular
• galbenă = feminine singular form of yellow
• nouă = feminine singular form of new

Compare:

• masculine singular: galben, nou
• feminine singular: galbenă, nouă

So the endings are there because the noun is feminine singular.

Because in Romanian, descriptive adjectives usually come after the noun.

So:

• lesa galbenă = the yellow leash

This is the normal Romanian pattern. English usually puts adjectives before the noun, but Romanian usually puts them after.

You can sometimes put an adjective before the noun in Romanian, but that is more marked, literary, or emphatic. The normal beginner pattern is:

• noun + adjective
• lesa galbenă

Because nouă is not just part of the noun phrase here. It is a predicate adjective after the verb a fi = to be.

Structure:

• Lesa galbenă = the subject
• este = is
• nouă = new

So the sentence is built like:

• The yellow leash
  • is
    • new

This is the same basic idea as English:

• The yellow leash is new

By contrast, galbenă directly modifies lesa inside the noun phrase:

• lesa galbenă = the yellow leash

Este is the 3rd person singular present form of a fi, which means to be.

So it can mean:

• is

In this sentence, it matches lesa = it:

• Lesa galbenă este nouă.
• The yellow leash is new.

A very common shorter spoken form is e:

• Lesa galbenă e nouă.

Both are correct; este is just the fuller form.

In normal standard Romanian, no. You normally keep the verb este or the shorter form e.

So you would say:

• Lesa galbenă este nouă. or
• Lesa galbenă e nouă.

Leaving it out completely would sound incomplete in an ordinary sentence.

Yes, Romanian has several forms spelled nouă.

It can mean:

• new (feminine singular adjective)
• nine
• to us

Here it means new, because the grammar makes that clear:

• it comes after este
• it agrees with lesa, a feminine singular noun

So in this sentence, nouă is definitely the adjective new, not the number or the pronoun form.

The dictionary forms are:

• lesă = leash
• galben = yellow
• nou = new
• a fi = to be

The sentence uses changed forms because Romanian marks things like definiteness and agreement:

• lesă → lesa
  attached definite article
• galben → galbenă
  feminine singular agreement
• nou → nouă
  feminine singular agreement
• a fi → este
  present tense, 3rd person singular

The letter ă is a separate Romanian vowel. It sounds like the a in the English weak final sound of sofa.

Very roughly:

• galbenă ≈ GAL-be-nuh
• nouă ≈ NO-wuh
• lesa ≈ LE-sa
• este ≈ ES-te

The exact Romanian sound is important, because ă is not the same as plain a.

You can, but it is not the most neutral everyday order.

The usual basic form is:

• lesa galbenă

If the adjective comes before the noun, Romanian often shifts the definite marking onto the adjective:

• galbena lesă

That kind of order can sound more emphatic, stylistic, or literary. For most everyday speech, a learner should treat lesa galbenă as the normal pattern.