Þessi villa kemur oft í eintölu, en ekki í fleirtölu.

Because villa is a feminine noun in Icelandic.

The demonstrative þessi changes for gender:

• þessi = masculine / feminine nominative singular
• þetta = neuter nominative singular

Since villa is feminine, you say þessi villa = this mistake/error.

If the noun were neuter, you would use þetta instead.

Villa means mistake, error, or sometimes fault, depending on context.

In this sentence, it most naturally means a mistake/error in language use, grammar, spelling, or something similar:

• Þessi villa = this error / this mistake

It is a common Icelandic noun, and its plural is villur.

Yes, kemur literally means comes, from the verb koma.

But Icelandic often uses koma in a broader way than English come. In this sentence, kemur oft í eintölu means something like:

• occurs often in the singular
• is often found in the singular
• shows up often in the singular

So although the literal meaning is comes, the natural meaning in English is closer to appears or occurs.

Kemur is the 3rd person singular present tense of koma.

So it is used with singular subjects like:

• hann kemur = he comes
• hún kemur = she comes
• þessi villa kemur = this error occurs/appears

The subject here, þessi villa, is singular, so kemur is the correct form.

Both eintölu and fleirtölu are in the dative singular because the preposition í often takes the dative when it means in in a static sense.

So:

• í eintölu = in the singular
• í fleirtölu = in the plural

The base forms are:

• eintala = singular
• fleirtala = plural

After í, they become:

• eintölu
• fleirtölu

This is a very common Icelandic pattern.

They are grammar terms:

• eintala = singular
• fleirtala = plural

Literally:

• ein- relates to one
• fleiri- / fleiri relates to more than one

So the sentence is talking about whether the error happens with singular forms or plural forms.

Icelandic often does not use the definite article in exactly the same places English does.

English says:

• in the singular
• in the plural

Icelandic says:

• í eintölu
• í fleirtölu

So even though English uses the, Icelandic does not need a separate article here. That is just the normal idiomatic way to express these grammar categories.

Oft means often.

In Icelandic, adverbs like oft commonly come after the verb:

• kemur oft = often occurs

So:

• Þessi villa kemur oft í eintölu
  literally: This error occurs often in singular

That word order is completely normal in Icelandic.

Because en ekki means but not.

The phrase:

• en ekki í fleirtölu

means:

• but not in the plural

Here, ekki negates the second part of the contrast. The sentence sets up a comparison:

• it happens often in the singular,
• but not in the plural.

This is a very common structure in Icelandic:

• X ..., en ekki Y = X ..., but not Y

No. En most often means but.

In this sentence, it clearly means but:

• ..., en ekki í fleirtölu = ..., but not in the plural

Icelandic og is the usual word for and.

So:

• og = and
• en = but

The structure is:

• Þessi villa = subject
• kemur = verb
• oft = adverb
• í eintölu = prepositional phrase
• en ekki í fleirtölu = contrasting phrase

So the sentence is built like this:

Subject + Verb + Adverb + Phrase + Contrast

That is a very normal Icelandic sentence pattern.

Yes, but it would mean something slightly different.

• oft = often
• oftast = most often

So:

• Þessi villa kemur oft í eintölu = This error often occurs in the singular
• Þessi villa kemur oftast í eintölu = This error occurs most often in the singular

The second version is stronger and more specific.

The plural would be:

• Þessar villur = these errors

Why?

• þessar is the feminine nominative plural form of þessi
• villur is the nominative plural of villa

So:

• Þessi villa = this error
• Þessar villur = these errors

In this sentence, þessi is a demonstrative determiner modifying the noun villa.

In simpler learner terms, it works like this in this error.

It agrees with the noun in gender, number, and case:

• feminine
• singular
• nominative

So it behaves a lot like an adjective in form, even though its function is specifically demonstrative.

A rough pronunciation guide for English speakers is:

• Þessi ≈ THESS-ih
• villa ≈ VIT-lah or VIL-lah depending on speech and accent
• kemur ≈ KYEH-mur
• oft ≈ oft
• í ≈ ee
• eintölu ≈ AYN-tuh-luh
• en ≈ ehn
• ekki ≈ EHK-kih
• í fleirtölu ≈ ee FLAIR-tuh-luh

A few useful notes:

• Þ is like English th in thin
• í is a long ee sound
• Icelandic r is usually trilled or tapped, not like most English r sounds

Exact pronunciation varies by speaker, but these approximations are a good start.