Ana mărește portretul.

A simple English-friendly approximation is:

• Ana → AH-na
• mărește → muh-RESH-te
• portretul → por-TRET-ul

A more precise guide:

• ă sounds like the a in about
• ș sounds like sh
• stress is usually:
  • A-na
  • mă-REȘ-te
  • por-TRET-ul

So the whole sentence is roughly:

AH-na muh-RESH-te por-TRET-ul

It comes from the infinitive a mări, which means to enlarge, to make bigger, or to increase.

So:

• a mări = to enlarge
• mărește = he/she enlarges or is enlarging

mărește is 3rd person singular, present tense.

That means it goes with:

• el mărește = he enlarges
• ea mărește = she enlarges

In your sentence, the subject is Ana, so mărește means Ana enlarges / is enlarging.

Because Romanian verbs change form when they are conjugated.

For a mări, some present-tense forms use -esc- / -eșt-:

• eu măresc = I enlarge
• tu mărești = you enlarge
• el/ea mărește = he/she enlarges
• noi mărim = we enlarge
• voi măriți = you all enlarge
• ei/ele măresc = they enlarge

So mărește is just the normal present-tense form for he/she.

-ul is the definite article attached to the noun. It means the.

So:

• portret = portrait
• portretul = the portrait

Romanian usually puts the at the end of the noun instead of using a separate word before it.

Because Romanian normally uses a postposed definite article: the article is attached to the end of the noun.

Compare:

• English: the portrait
• Romanian: portretul

This is very common in Romanian:

• băiat = boy → băiatul = the boy
• carte = book → cartea = the book
• portret = portrait → portretul = the portrait

The noun portret is neuter in Romanian.

Neuter nouns in Romanian behave:

• like masculine in the singular
• like feminine in the plural

So you get:

• singular indefinite: un portret
• singular definite: portretul
• plural indefinite: două portrete
• plural definite: portretele

Because Ana is a proper name, and proper names usually do not take an article in this kind of sentence.

So Romanian says simply:

• Ana mărește portretul

not something like the Ana.

The basic, neutral order is:

Subject + Verb + Object
Ana mărește portretul.

That is the most natural order for a simple statement.

Romanian word order is more flexible than English, but changing the order usually changes the emphasis. So this version is the straightforward, neutral one.

Yes. Romanian often drops the subject pronoun or noun if the context already makes it clear.

So you could say:

• Mărește portretul.

This would mean He/She is enlarging the portrait or enlarges the portrait, depending on context.

In your sentence, Ana is included to make it clear exactly who is doing the action.

It can mean either one, depending on context.

Romanian present tense often covers both:

• Ana enlarges the portrait
• Ana is enlarging the portrait

If there is no extra context, English might translate it either way.

Because portretul is an inanimate direct object.

Romanian often uses pe with specific people or sometimes animate beings as direct objects, but normally not with things.

For example:

• Ana o vede pe Maria. = Ana sees Maria.
  Here pe is used because Maria is a person.

But:

• Ana mărește portretul.
  No pe, because portretul is a thing, not a person.

Yes, very important.

In mărește:

• ă is a real Romanian letter
• ș is a real Romanian letter

They change pronunciation and are part of the standard spelling. Writing mareste without diacritics is common in informal typing sometimes, but the correct form is mărește.