Ég set kommu á réttan stað og punkt í lok setningarinnar.

Setja is the infinitive, meaning to put / to place. In the sentence, the subject is ég (I), so the verb has to be in the present tense, 1st person singular: ég set.

A quick comparison:

• að setja = to put
• ég set = I put / I am putting
• þú setur = you put
• hann/hún setur = he/she puts

So set is just the normal form that goes with ég.

Because they are direct objects of the verb set, and setja takes the accusative.

So:

• komma → kommu
• punktur → punkt

The two nouns belong to different declension patterns, so they change differently:

• komma is a weak feminine noun, so it becomes kommu in the accusative singular.
• punktur is a masculine noun, and many masculine nouns lose -ur in the accusative singular, giving punkt.

Because Icelandic does not have an indefinite article like English a/an.

A bare singular noun often translates as a/an in English:

• kommu = a comma
• punkt = a period / a full stop

If you want the, Icelandic usually adds it to the end of the noun:

• komman = the comma
• punkturinn = the period

This is a very common Icelandic pattern: with many prepositions, the case changes depending on whether you mean movement or location.

With á:

• accusative often suggests movement to a place or position
• dative often suggests being in/on a place or position

Here, setja means placing something somewhere, so there is movement into position:

• á réttan stað = to the right place / into the correct position

By contrast:

• á réttum stað = in the right place as a state or location

Also, English says in the right place, but Icelandic uses á here. Prepositions do not always match one-to-one across the two languages.

Both words are masculine singular accusative.

Breakdown:

• staður = nominative singular
• stað = accusative singular
• réttur = nominative singular masculine adjective
• réttan = accusative singular masculine adjective

The adjective must agree with the noun in gender, number, and case, so:

• réttan stað = adjective + noun, both masculine singular accusative

Because á stað is the natural Icelandic way to talk about putting something in its proper place or position.

So even though English says:

• in the right place

Icelandic says:

• á réttan stað

This is just one of those places where you have to learn the Icelandic preposition as part of the expression, rather than translating the English preposition directly.

Í lok is a very common fixed expression meaning at the end of.

So:

• í lok setningarinnar = at the end of the sentence

The important grammar point is that lok is followed by a noun in the genitive:

• setningarinnar = of the sentence

So you can think of it as:

• í lok + genitive noun

Because it contains several pieces of grammar at once.

The base noun is:

• setning = sentence

Its genitive singular form is:

• setningar = of a sentence

Then Icelandic adds the suffixed definite article:

• setningarinnar = of the sentence

So this one word includes:

• the noun stem
• the genitive ending
• the definite article

That is very normal in Icelandic.

Because Icelandic, like English, can use one verb for two coordinated items joined by og (and).

So the sentence means:

• Ég set kommu á réttan stað og punkt í lok setningarinnar
• literally: I put a comma in the right place and a period at the end of the sentence

The verb set applies to both nouns:

• set kommu ...
• (set) punkt ...

Repeating the verb would usually be unnecessary unless you wanted extra emphasis or clarity.

The sentence follows the normal main-clause pattern:

• Ég = subject
• set = finite verb
• then the rest of the sentence

So:

• Ég set kommu á réttan stað og punkt í lok setningarinnar

This also fits Icelandic’s V2 tendency, where the finite verb normally comes in the second position of a main clause. Since the subject Ég is first, the verb set comes right after it.