Min allergi blir värre på morgonen, men på kvällen finns det mindre pollen.

Because allergi is a common-gender noun in Swedish, not a neuter noun.

• en allergi = an allergy
• common-gender nouns take min
• neuter nouns take mitt

So:

• min allergi = my allergy

If it were a plural noun, you would use mina.

Swedish often uses a possessive with health conditions, body parts, and personal states when talking about your own specific case of something.

So min allergi blir värre means my allergy gets worse and sounds very natural.

You could also say jag har allergi, but that means I have an allergy / allergies, which is a different idea. In this sentence, the focus is not on having the allergy, but on how it changes.

Because blir means becomes / gets, while är means is.

• blir värre = gets worse / becomes worse
• är värre = is worse

In this sentence, the allergy is changing over time, especially in the morning, so blir is the natural choice.

Compare:

• Min allergi blir värre på morgonen. = My allergy gets worse in the morning.
• Min allergi är värre än din. = My allergy is worse than yours.

Värre is the comparative form, meaning worse.

This is an irregular pattern:

• dålig = bad
• sämre = worse, better-known when talking about quality in general

But with states like feeling unwell or things getting worse, Swedish also commonly uses:

• illa / related pattern → värre = worse

In everyday Swedish, värre is very common in expressions like:

• Det blir värre. = It gets worse.
• Jag mår värre idag. = I feel worse today.

So here blir värre is a very natural fixed expression: gets worse.

In Swedish, parts of the day are very often used with the definite form in time expressions:

• på morgonen = in the morning
• på kvällen = in the evening
• på eftermiddagen = in the afternoon
• på natten = at night / during the night

So even though English often uses in the morning without a visible definite ending on the noun, Swedish builds that idea into the noun itself:

• morgon → morgonen
• kväll → kvällen

This is standard and very common.

With times of day, Swedish commonly uses på in expressions like these:

• på morgonen
• på dagen
• på eftermiddagen
• på kvällen

So you should usually learn them as fixed time phrases.

Even though English says in the morning and in the evening, Swedish uses på, not i, in these common expressions.

This is because Swedish follows the verb-second rule in main clauses.

When a sentence starts with something other than the subject, the finite verb still has to come in the second position.

Normal order:

• Det finns mindre pollen på kvällen.

If you move på kvällen to the front, the verb must still stay second:

• På kvällen finns det mindre pollen.

So the order becomes:

1. på kvällen
2. finns
3. det
4. mindre pollen

This is one of the most important word-order rules in Swedish.

Det finns means there is / there are.

It is the standard Swedish way to say that something exists or is present somewhere.

• Det finns pollen. = There is pollen. / There are pollen particles.
• Det finns mindre pollen. = There is less pollen.

In your sentence, det is not the same as English it in meaning. It is part of the structure used for there is / there are.

Because på kvällen is moved to the front, the normal det finns becomes finns det because of Swedish word order:

• Det finns mindre pollen på kvällen.
• På kvällen finns det mindre pollen.

Because pollen is being treated as an uncountable substance here, similar to dust, water, or smoke.

• mindre = less
• färre = fewer

So:

• mindre pollen = less pollen

Use färre with countable plural nouns:

• färre bilar = fewer cars
• färre människor = fewer people

Use mindre with uncountable nouns:

• mindre vatten = less water
• mindre socker = less sugar
• mindre pollen = less pollen

Here, pollen is best understood as a mass noun or uncountable noun. It refers to the substance in general, not to separate individual units.

So in this sentence, you should think of it like:

• water
• dust
• smoke

That is why there is no article before it and why Swedish uses mindre.

Yes. That version is completely natural.

• Det finns mindre pollen på kvällen.
• På kvällen finns det mindre pollen.

Both mean the same thing.

The difference is emphasis:

• Det finns mindre pollen på kvällen = more neutral
• På kvällen finns det mindre pollen = puts extra focus on in the evening

Swedish often moves time expressions to the front when they are important to the message.

Because men means but, and it joins two main clauses:

• Min allergi blir värre på morgonen
• på kvällen finns det mindre pollen

In Swedish, it is very common to put a comma before men when it connects full clauses. This helps show the contrast clearly.

So the comma is natural and standard here.

A few words may stand out:

• allergi — stress on the last syllable: alle-r-gi
• värre — the ä is like a broad vowel, and the word is short and strong
• morgonen — stress on mor
• kvällen — stress on kväll
• finns — short, with a clear final s
• mindre — stress on min

A useful rhythm for the whole sentence is:

Min allergi blir VÄRre på MORgonen, men på KVÄLlen finns det MINDre POLlen.

Even if your exact accent is not perfect, getting the stress in roughly the right places will make you easier to understand.