Sein Parfüm riecht angenehm, aber heute ist es ein bisschen zu stark.

Because Parfüm is a neuter singular noun: das Parfüm.

The possessive sein- here means his. The ending on a possessive changes to match the noun that follows it. In the nominative singular neuter, it usually has no extra ending:

• sein Parfüm = his perfume
• compare:
  • seine Jacke = his jacket
  • sein Auto = his car

So sein is correct because the noun after it is neuter singular.

In a way, it does both:

• the base form sein- tells you about the owner: his
• the ending tells you about the noun that follows

So in sein Parfüm:

• sein- = belonging to him
• zero ending = because Parfüm is nominative singular neuter

This is similar to how English says his car, his jacket, his house, but German changes the form depending on the noun.

Because riechen means to smell.

So:

• Das Parfüm riecht angenehm = the perfume smells pleasant
• Das Parfüm ist angenehm = the perfume is pleasant

Both can be possible, but they are not exactly the same.

• riecht angenehm focuses specifically on the smell
• ist angenehm is broader and means it is pleasant in general

With perfume, talking about the smell is the most natural choice, so riecht angenehm fits very well.

Because angenehm is not being used directly before a noun here. It is used predicatively/adverbially after the verb riecht.

Compare:

• ein angenehmes Parfüm = an agreeable / pleasant perfume
  
  • here the adjective is before a noun, so it takes an ending
• Das Parfüm riecht angenehm = the perfume smells pleasant
  
  • here it does not take an ending

This is similar to:

• Das Essen ist gut
• Er singt schön
• Die Blumen riechen wunderbar

No, they are different.

• sehr stark = very strong
• zu stark = too strong

So zu expresses that something goes beyond what is wanted or comfortable.

In this sentence:

• heute ist es ein bisschen zu stark
  = today it is a little too strong

That suggests the perfume is normally fine, but today it feels excessive.

Ein bisschen means a little / a bit, and here it modifies zu stark.

So the structure is:

• ein bisschen zu stark = a little too strong

This is a very common German pattern:

• ein bisschen zu laut = a little too loud
• ein bisschen zu teuer = a little too expensive
• ein bisschen zu spät = a little too late

It softens the statement, making it sound less harsh.

Es refers back to sein Parfüm.

Since das Parfüm is neuter, the pronoun used for it is es:

• das Parfüm → es

So the second clause could be expanded as:

• aber heute ist das Parfüm ein bisschen zu stark

But German, like English, normally uses a pronoun instead of repeating the noun.

Both are possible, but they emphasize things slightly differently.

In German main clauses, the finite verb is in second position. The first position can be taken by different elements.

Here:

• Aber heute ist es ein bisschen zu stark.

The first position is heute, so the verb ist comes right after it.

This gives a natural emphasis to today:

• but today it is a bit too strong

You can also say:

• Aber es ist heute ein bisschen zu stark.

That is also grammatical, but it sounds a little less focused on today.

Because aber connects two main clauses, and in German, main clauses joined by conjunctions like aber are usually separated by a comma.

Here the two clauses are:

• Sein Parfüm riecht angenehm
• aber heute ist es ein bisschen zu stark

So the comma is standard German punctuation.

Yes. Riechen is very flexible in German.

It can mean:

1. to smell in the sense of giving off a smell

   • Das Parfüm riecht angenehm.
   • The perfume smells pleasant.

2. to smell of / like

   • Es riecht nach Kaffee.
   • It smells like coffee.

3. to smell in the sense of perceiving with the nose

   • Ich rieche das Parfüm.
   • I smell the perfume.

So learners often need to rely on context to know which meaning is intended.

Yes, but it changes the tone a little.

• riecht = smells
• duftet = smells nice / gives off a pleasant fragrance

So:

• Sein Parfüm riecht angenehm = neutral and natural
• Sein Parfüm duftet angenehm = a bit more literary or elegant

Because the second half says the perfume is too strong today, riecht works especially well: it stays neutral rather than sounding too flowery.

Yes, Parfüm is a normal German word for perfume.

It is pronounced approximately like par-FYM, with stress on the second syllable. The ü sound does not exist exactly in English, so learners often need practice with it.

Also remember:

• singular: das Parfüm
• plural: die Parfüms

So this sentence uses a very standard everyday noun.