Des de fa tres dies, vull collir les flors del jardí, però encara són petites.

It means for the last three days or for three days now.

Catalan uses des de fa + period of time to talk about something that started in the past and is still true now.

So:

• Des de fa tres dies = for three days
• Des de fa una setmana = for a week
• Des de fa molt temps = for a long time

In this sentence, it tells you that the speaker’s wish started three days ago and continues up to the present.

Because the two expressions do different jobs:

• fa tres dies usually means three days ago
• des de fa tres dies means for the last three days

Compare:

• Fa tres dies vaig plantar les flors.
  = I planted the flowers three days ago.

• Des de fa tres dies, vull collir les flors.
  = I have wanted / I’ve been wanting to pick the flowers for three days.

So des de fa shows duration up to now.

Because in Catalan, the present tense is often used for something that began in the past and still continues now, especially with expressions like des de fa tres dies.

So:

• Des de fa tres dies, vull collir les flors...

is very naturally understood as:

• I’ve wanted to pick the flowers for three days
• I’ve been wanting to pick the flowers for three days

English often prefers a perfect or perfect continuous idea here, but Catalan commonly uses the present.

Collir means to pick, to gather, or to harvest, especially things like:

• flowers
• fruit
• vegetables

So collir les flors means to pick the flowers.

It is more specific than a general verb like agafar:

• collir una flor = pick a flower
• agafar una flor = take/grab a flower

If you are talking about flowers, fruit, or crops, collir is the natural choice.

Del is the contraction of:

• de + el = del

So:

• les flors del jardí
  literally = the flowers of the garden

In natural English, that usually becomes:

• the flowers in the garden
• the garden’s flowers

This kind of contraction is very common in Catalan.

Catalan uses definite articles very naturally in places where English may or may not use them.

Here:

• les flors = the flowers
• el jardí = the garden
• del jardí = of the garden / in the garden

The speaker is talking about specific flowers: the ones from that garden. So the articles are completely normal.

Here encara means still.

So:

• encara són petites = they are still small

It shows that the situation has not changed yet.

A very useful contrast is:

• encara = still
• encara no = not yet

Examples:

• Encara plou. = It’s still raining.
• Encara no plou. = It isn’t raining yet.

Because ser is the normal verb with petit/petita when you are describing size.

So:

• són petites = they are small

In Catalan, ser is commonly used for qualities like size, shape, identity, and classification.

Here the idea is that the flowers are small in size, so són petites is the natural choice.

A learner-friendly rule is:

• use ser for descriptions like small, big, beautiful, red
• use estar more often for states or conditions like open, closed, tired, ready

Because adjectives in Catalan agree with the noun they describe.

Here the noun is:

• flors = feminine plural

So the adjective must also be feminine plural:

• petita = feminine singular
• petites = feminine plural

Compare:

• la flor petita = the small flower
• les flors petites = the small flowers

Yes. A very common alternative is:

• Fa tres dies que vull collir les flors del jardí, però encara són petites.

This also means I’ve been wanting to pick the flowers in the garden for three days, but they’re still small.

The original version with Des de fa tres dies is also correct. Putting that phrase at the beginning gives the time frame first, and the comma helps separate it from the rest of the sentence.

Yes, they are important.

• però = but
• són = they are

Accent marks in Catalan help show correct stress and sometimes distinguish words from others that are spelled similarly.

For example:

• són = are
• son can refer to sleep

So you should learn the accents as part of the word.

Però means but, and it is correct here because the second part simply contrasts with the first:

• I want to pick the flowers, but they are still small.

Sinó is used after a negative statement to correct or replace something.

Compare:

• Vull collir les flors, però encara són petites.
  = I want to pick the flowers, but they are still small.

• No són grans, sinó petites.
  = They aren’t big, but rather small.

So in your sentence, però is the right conjunction.