Aleg unu.

Aleg is the 1st person singular present tense of the verb a alege, meaning to choose or to pick.

So:

• a alege = to choose
• aleg = I choose

Romanian often uses one-word verb forms that already show the subject, so aleg by itself already means I choose.

Romanian usually drops the subject pronoun when it is clear from the verb ending.

So:

• Eu aleg = I choose
• Aleg = also I choose

Both are possible, but Aleg is more natural unless you want emphasis:

• Eu aleg, nu tu. = I’m choosing, not you.

Un usually works like a/an before a noun:

• un băiat = a boy
• un măr = an apple

But here there is no noun after it. The idea is simply one.

That is why you do not use un here. Instead, Romanian uses the numeral/pronominal form unu or, more standardly in this kind of sentence, unul.

A learner will very often meet Aleg unul. as the more standard form for I choose one.

Why?

• unul is the usual pronoun meaning one
• it stands in for a noun that is not repeated

So:

• Aleg unul. = I choose one.

Aleg unu. can be heard in informal speech, especially in quick conversation, but unul is the safer form to learn for standard Romanian.

It is acting like a pronoun: it replaces a noun that is understood from context.

For example:

• Vrei mărul roșu sau mărul verde?
• Aleg unul.

Here unul means something like one of them or one one in English, depending on context.

So in Aleg unul, the noun does not need to be repeated because unul stands in for it.

A simple learner-friendly pronunciation is:

• Aleg ≈ ah-LEG
• unu ≈ OO-noo

A few notes:

• the g in aleg is a hard g, like in go
• the stress is on the second syllable: a-LEG
• u in Romanian is usually a clear oo sound

If you use the more standard unul, that is approximately:

• OO-nool

Yes. Eu aleg unul is grammatically fine.

But in normal Romanian, the eu is often omitted unless you want contrast or emphasis.

Compare:

• Aleg unul. = neutral, normal
• Eu aleg unul. = emphasizes I

Yes.

Romanian distinguishes masculine and feminine here:

• unul = masculine one
• una = feminine one

Examples:

• Aleg unul. = I choose one. (masculine thing)
• Aleg una. = I choose one. (feminine thing)

So the form depends on the noun being replaced.

Yes, but Aleg unul is the normal, straightforward order.

Romanian word order is somewhat flexible, but changes usually add emphasis or sound marked.

For example:

• Aleg unul. = neutral
• Unul aleg. = unusual, emphatic, something like It’s one that I’m choosing

For learners, the standard pattern verb + object is best here.

Yes, it can be a complete sentence, especially in context.

Romanian often allows short sentences like this when the missing noun is understood:

• looking at several options: Aleg unul.
• choosing among objects: Aleg una.

So even though English learners may expect a repeated noun, Romanian does not need it here.