Эта роза пахнет очень приятно.

Because роза is a feminine singular noun, and the demonstrative this has to agree with it.

• этот = masculine
• эта = feminine
• это = neuter

So:

• этот стол = this table
• эта роза = this rose
• это письмо = this letter

Since роза is feminine, эта is the correct form.

Here роза is the subject of the sentence, so it is in the nominative case.

The basic dictionary form is:

• роза = rose

In Эта роза пахнет очень приятно, the sentence is about what the rose does, so роза stays in the nominative.

Пахнет is the 3rd person singular present-tense form of пахнуть, which means to smell / to give off a smell.

So literally:

• роза пахнет = the rose smells

In this sentence, it means the rose has a pleasant smell or gives off a pleasant scent.

No. English smell can mean two different things:

1. to give off a smell
   
   • The rose smells nice.
2. to sense an odor with your nose
   
   • I smell smoke.

Russian usually separates these ideas more clearly.

• пахнуть = to smell, to give off a scent
  
  • Роза пахнет приятно.
• нюхать = to sniff
  
  • Он нюхает розу. = He is sniffing the rose.
• чувствовать запах = to smell/sense a smell
  
  • Я чувствую запах дыма. = I smell smoke.

So in your sentence, пахнет means gives off a smell, not sniffs or detects a smell.

Because приятно is an adverb, and it describes how the rose smells.

• приятная is an adjective: pleasant (feminine)
• приятно is an adverb: pleasantly

So:

• приятная роза = a pleasant rose
• роза пахнет приятно = the rose smells pleasantly

In this sentence, we are not directly describing the rose as a noun; we are describing the action/state expressed by пахнет.

Очень means very. It intensifies приятно.

So:

• приятно = pleasantly / nice
• очень приятно = very pleasantly / very nice

In natural English, you would usually translate the whole phrase more smoothly as:

• This rose smells very nice.
• This rose smells very pleasant.

Yes, Russian word order is fairly flexible.

The neutral order here is:

• Эта роза пахнет очень приятно.

But you could also hear:

• Эта роза очень приятно пахнет.

That also sounds natural.

Moving words around can slightly change emphasis, but the basic meaning stays the same because the grammar endings still show what each word is doing.

It can mean either, depending on context.

• right now: this rose smells nice at the moment
• general quality: this rose is a pleasantly scented kind of rose

Russian present tense often covers both meanings unless the context makes one interpretation clearer.

A helpful stress guide is:

• Э́та ро́за па́хнет о́чень прия́тно.

A rough pronunciation for an English speaker:

• EH-ta RO-za PAKH-nyet O-chin pri-YAT-na

A few notes:

• х is like the ch in German Bach or Scottish loch, not like English h
• пахнет has a consonant cluster, and the н is clearly pronounced: па́х-нет
• очень ends with a soft нь

Yes. You can simply say:

• Роза пахнет очень приятно.

That means The rose smells very pleasant / Roses smells very nice, depending on context.

Adding эта makes it more specific:

• Эта роза... = This rose...

So эта helps point to a particular rose.

Because Russian normally does not use a present-tense verb meaning is in sentences like this.

In English, we often need is, but Russian usually does not.

However, in this particular sentence, the main verb is already пахнет (smells), so there is no need for any extra verb anyway.

The sentence is complete with just:

• subject: Эта роза
• verb: пахнет
• adverb phrase: очень приятно

Yes. That is very common and natural.

• Эта роза пахнет очень приятно. = This rose smells very pleasant.
• Эта роза хорошо пахнет. = This rose smells good.

The version with приятно sounds a bit more literally like pleasantly; the version with хорошо is often more everyday and idiomatic.

Both are correct.

The dictionary form is пахнуть.

Present tense:

• я пахну = I smell / I give off a smell
• ты пахнешь
• он / она пахнет
• мы пахнем
• вы пахнете
• они пахнут

In your sentence, the subject is роза (she/it grammatically singular), so the correct form is пахнет.