Pun scrisoarea într-un plic alb.

Pun is the 1st person singular present tense of a pune (to put).

So Pun scrisoarea... means I put the letter... or, depending on context, I am putting the letter...

Romanian often does not need the subject pronoun eu (I) because the verb ending already shows who is doing the action.
So:

• Pun = I put
• Eu pun = I put (with extra emphasis on I)

Romanian is a pro-drop language, which means subject pronouns are often omitted when they are clear from the verb form.

Since pun already means I put, adding eu is usually unnecessary unless you want contrast or emphasis:

• Pun scrisoarea... = neutral, normal
• Eu pun scrisoarea... = I am putting the letter (not someone else)

Scrisoarea means the letter, while scrisoare means a letter / letter.

Romanian usually puts the definite article at the end of the noun, not before it as English does.

So:

• scrisoare = a letter
• scrisoarea = the letter

In this sentence, the speaker is referring to a specific letter, so scrisoarea is used.

That is a normal feature of Romanian grammar. Romanian has a postposed definite article, meaning the word for the is usually attached to the end of the noun.

Examples:

• băiat = boy

• băiatul = the boy

• plic = envelope

• plicul = the envelope

• scrisoare = letter
• scrisoarea = the letter

So instead of a separate word like English the, Romanian often adds an ending.

Într-un means in a or into a.

It is basically a contracted form of:

• în = in / into
• un = a

So:

• într-un plic = in an envelope / into an envelope

Romanian often uses într-un before an indefinite masculine/neuter singular noun.

You may also see:

• într-o = in a / into a (before feminine singular nouns)

For example:

• într-un plic = into an envelope
• într-o cutie = into a box

Not exactly.

• în means in / into
• într-un means in a / into a

So într-un includes both the preposition and the indefinite article.

Compare:

• Pun scrisoarea în plic. = I put the letter in the envelope / in envelope-like context
• Pun scrisoarea într-un plic. = I put the letter in an envelope

The sentence with într-un clearly introduces an envelope.

Because the meaning here is a white envelope, not the white envelope.

• un plic alb = a white envelope
• plicul alb = the white envelope

So the sentence contrasts:

• scrisoarea = the letter (specific)
• un plic alb = a white envelope (not specified)

This mix of definite and indefinite nouns is completely normal.

In Romanian, adjectives often come after the noun.

So:

• plic alb = white envelope
• literally: envelope white

This is one of the most common adjective positions in Romanian.

You can sometimes place adjectives before the noun, but that often changes the style, emphasis, or tone. For normal everyday description, noun + adjective is very common.

Alb agrees with plic, which is masculine singular in this sentence.

Agreement here is:

• plic alb = masculine singular
• scrisoare albă = feminine singular
• plicuri albe = plural
• scrisori albe = plural

So alb is the correct form because plic is masculine singular.

Plic is a neuter noun in Romanian.

Romanian neuter nouns behave:

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

So in the singular:

• un plic alb = a white envelope

But in the plural:

• două plicuri albe = two white envelopes

That is why the adjective here looks masculine singular: alb.

It can mean either one, depending on context.

Romanian present tense often covers both:

• a habitual/simple present meaning: I put
• an ongoing/present progressive meaning: I am putting

If you want to strongly emphasize the ongoing action, Romanian may use context or another construction, but the simple present is very commonly enough.

In this sentence, English usually prefers into, because there is movement:

• Pun scrisoarea într-un plic alb. = I put the letter into a white envelope.

But Romanian often uses în where English distinguishes between in and into.

So Romanian în can cover both ideas, and context tells you whether it is location or movement.

Yes, Romanian word order is somewhat flexible, but the neutral order here is very natural:

• Pun scrisoarea într-un plic alb.

This is the most straightforward way to say it.

Other orders are possible for emphasis, but they may sound marked or literary depending on context. For a learner, the basic pattern is best:

verb + direct object + prepositional phrase

A helpful rough guide is:

• scrisoarea ≈ skree-soa-rea

The final -a is part of the definite article.
Also, Romanian ea often forms a glide-like sequence rather than two fully separate English-style vowels.

A more important learner point is to recognize that:

• scrisoare = letter
• scrisoarea = the letter

even if the pronunciation changes slightly when the article is added.

Yes. It is written with a hyphen:

• într-un
• într-o

This is the standard spelling. It reflects the fused form used in modern Romanian.

So you should learn it as a normal written pattern, not as two completely separate words in this construction.