Það kom blettur á gólfið, því málningin datt af penslinum.

Here það is a dummy subject or expletive, much like English there in There appeared a stain on the floor.

So in Það kom blettur á gólfið, það does not refer to a thing. It is just there to fill the first position in the sentence.

A very natural way to understand the structure is:

• Það kom blettur ... = There came / appeared a stain ...
• more idiomatic English: A stain got onto the floor or There was a stain on the floor

This use of það is very common in Icelandic when introducing something new or indefinite.

Icelandic is a verb-second language, which means the finite verb usually comes in the second position of a main clause.

In this sentence:

• Það is in the first position
• kom is therefore in the second position
• blettur comes after the verb

So:

• Það kom blettur á gólfið is structurally very normal

If you said Blettur kom á gólfið, that is possible grammatically, but it sounds more marked and gives blettur more emphasis. The version with það is more natural for introducing a new thing into the discourse.

Because blettur is the grammatical subject of the clause.

Even though English would often say there was a stain, Icelandic still treats blettur as the real subject, so it appears in the nominative:

• blettur = nominative singular of a masculine noun

The það at the beginning is only a placeholder, not the true subject.

Because the sentence is introducing a stain, not the stain.

In Icelandic, the definite article is usually attached to the end of the noun:

• blettur = a stain
• bletturinn = the stain

Since this is new information, the indefinite form blettur is the natural choice.

This is a very important Icelandic case pattern.

With á:

• á + accusative often means movement to / onto
• á + dative often means location on

So:

• á gólfið = onto the floor
• á gólfinu = on the floor

Here the idea is that the stain came onto the floor, so Icelandic uses the accusative:

• gólfið = accusative singular of gólf

This is one of the most common contrasts learners need to master.

Yes, but it would change the nuance.

• Það kom blettur á gólfið focuses on the stain appearing on / getting onto the floor.
• Það var blettur á gólfinu would mean There was a stain on the floor, focusing on the stain’s presence there.
• Blettur var á gólfinu is also possible, though less neutral in tone.

So á gólfið matches the event of paint landing there, while á gólfinu would describe the result or location.

Here því means because.

It introduces the reason:

• því málningin datt af penslinum
• because the paint fell off the brush

You may also see því að, which is very common:

• ..., því að málningin datt af penslinum.

In many contexts, the að can be omitted, especially in ordinary writing and speech.

Because því introduces an explanatory subordinate clause.

Icelandic punctuation often uses a comma before clauses like this, especially when they give a reason or explanation:

• Það kom blettur á gólfið, því málningin datt af penslinum.

This comma is normal and helps separate the main statement from the reason.

Because málningin means the paint.

The ending -in is the definite article attached to the noun:

• málning = paint
• málningin = the paint

Using the definite form suggests we are talking about a specific paint already understood from the situation, for example the paint on the brush.

Also note that málning is a feminine noun, and that affects its forms.

Datt is the past tense of the verb detta, meaning to fall.

So:

• detta = to fall
• datt = fell

This is a strong verb, so the past tense is not formed by just adding a regular ending.

Examples:

• Glasið datt. = The glass fell.
• Málningin datt af penslinum. = The paint fell off the brush.

Because the preposition af governs the dative case.

The noun is pensill (brush), and its dative singular definite form is:

• penslinum = from/off the brush

So:

• af penslinum = off the brush

This is a very common Icelandic pattern:

• af + dative

For example:

• af borðinu = off the table
• af veggnum = off the wall

Because it means the brush, not just a brush.

Compare:

• af pensli = off a brush
• af penslinum = off the brush

The sentence refers to a particular brush involved in the situation, so the definite form is natural.

The nouns here belong to different grammatical genders:

• blettur = masculine
• málning = feminine
• gólf = neuter
• pensill = masculine

This matters because gender affects:

• articles
• adjective agreement
• case endings
• some pronouns

For example:

• blettur → bletturinn
• málning → málningin
• gólf → gólfið
• pensill → pensillinn

And when they change case, the endings change differently depending on gender and noun class.

Yes. Icelandic often uses verbs like koma in ways that sound a little unusual in English.

Literally, kom blettur is came a stain, but the natural sense is:

• a stain appeared
• a stain got onto the floor
• there was a stain on the floor in context

So although the verb is koma (come), the overall expression is idiomatic and natural in Icelandic.

It breaks down like this:

Það kom blettur á gólfið, því málningin datt af penslinum.

• Það = dummy subject
• kom = past tense verb
• blettur = subject
• á gólfið = direction/result: onto the floor
• því = because
• málningin = the paint
• datt = fell
• af penslinum = off the brush

So the pattern is:

• Main clause: something happened
• Reason clause: why it happened

That is a very common sentence type in Icelandic.