På kvällen finns det ofta fler myggor nära sjön än vid huset.

In Swedish, på kvällen is the normal way to say in the evening / during the evening.

A useful pattern is:

• på morgonen = in the morning
• på dagen = in the daytime / during the day
• på kvällen = in the evening
• på natten = at night

So even though English uses in, Swedish usually uses på in these expressions.

Kvällen is the definite form of kväll (evening).

In time expressions like på kvällen, Swedish often uses the definite form where English uses a more general phrase:

• på kvällen = in the evening
• på sommaren = in the summer
• på vintern = in the winter

It does not necessarily mean one specific evening. It can also mean evenings in general, depending on context.

This is because of Swedish V2 word order in main clauses.

In a Swedish main clause, the finite verb usually comes in the second position. Since the sentence starts with På kvällen, that counts as the first element, so the verb finns must come next:

• På kvällen finns det ofta fler myggor ...

If the sentence started with det, then you would get:

• Det finns ofta fler myggor på kvällen ...

Both are possible, but the word order changes depending on what comes first.

Here, det is an expletive or dummy subject, similar to English there in there is / there are.

So:

• Det finns en bok på bordet. = There is a book on the table.
• Det finns många myggor här. = There are many mosquitoes here.

Even though det usually means it, in this structure it does not refer to a specific thing. It is just part of the existential expression det finns = there is/are.

Because myggor is a countable noun: you can count mosquitoes.

Swedish normally uses:

• fler = more, with countable nouns
• mer = more, with uncountable nouns or sometimes adjectives/adverbs

So:

• fler myggor = more mosquitoes
• mer vatten = more water
• mer intressant = more interesting

So fler myggor is the correct choice.

They are related, but they are not used the same way.

• fler = more
• flera = several

Examples:

• fler myggor än ... = more mosquitoes than ...
• flera myggor = several mosquitoes

In your sentence, there is a comparison with än (than), so fler is the right form.

Än is used in comparisons and means than.

So:

• fler myggor nära sjön än vid huset = more mosquitoes near the lake than by the house

This is the normal comparative pattern:

• större än = bigger than
• yngre än = younger than
• fler än = more than

Both describe location, but they are slightly different in feel.

• nära = near, close to
• vid = by, beside, at

So:

• nära sjön = near the lake
• vid huset = by the house / at the house

In many contexts they can both sound natural, but nära emphasizes closeness, while vid often suggests a position right by something.

Because the sentence refers to the lake and the house as known or identifiable places.

• sjö = a lake
• sjön = the lake
• hus = a house / house
• huset = the house

Swedish often uses the definite form when talking about places that are understood from context, just like English uses the.

Also note the definite endings:

• common gender noun: sjö → sjön
• neuter noun: hus → huset

In this sentence, ofta comes after finns det:

• På kvällen finns det ofta fler myggor ...

This is a very normal position for a sentence adverb or frequency adverb in Swedish. After the finite verb and subject/dummy subject, adverbs like ofta commonly appear before the rest of the sentence.

Compare:

• Det finns ofta myggor här.
• Hon kommer ofta hit.

Adverb placement can vary a little depending on emphasis, but this placement is standard and natural.

Yes, that is grammatically possible, but it gives a slightly different flow.

• På kvällen finns det ofta fler myggor ... puts focus on when
• Det finns ofta fler myggor ... på kvällen starts more neutrally and leaves the time expression until the end

Swedish often moves the part you want to emphasize to the front of the sentence.

Yes, myggor is the indefinite plural of mygga.

• en mygga = a mosquito
• myggor = mosquitoes
• myggorna = the mosquitoes

Swedish does not use an article in the indefinite plural. So myggor by itself means mosquitoes.

That is why:

• fler myggor = more mosquitoes

not something like fler några myggor or fler en myggor.

It can often mean in the evening / during the evening, and depending on context it can also have a more general meaning like in the evenings.

In your sentence, especially with ofta, it sounds like a general statement:

• På kvällen finns det ofta fler myggor ...
• In the evening(s), there are often more mosquitoes ...

So Swedish often uses the singular definite form in a way that English might translate more generally.