Ég er nýbúin að setja mold í beðið.

Because this sentence uses the Icelandic pattern vera nýbúinn að + infinitive, which means to have just finished doing something or to have just done something.

So:

• Ég er nýbúin að setja ... = I have just put ...
• literally, it is closer to I am newly done with putting ...

This is a very common Icelandic way to express a recent completed action. English usually uses have here, but Icelandic uses be in this expression.

Nýbúin is the feminine singular form of nýbúinn, which means something like:

• just finished
• having just done
• recently done

In this sentence, nýbúin að setja means having just put.

This word changes form to agree with the person being talked about:

• Ég er nýbúinn ... — said by a man
• Ég er nýbúin ... — said by a woman
• Það er nýbúið ... — neuter

So the sentence strongly suggests that the speaker is female.

Because Icelandic adjectives and participle-like forms agree in gender and number.

Here, the subject is ég (I), and the form nýbúin tells us the speaker is feminine singular.

Compare:

• Ég er nýbúinn að setja mold í beðið. — a male speaker
• Ég er nýbúin að setja mold í beðið. — a female speaker

English does not show this kind of agreement, but Icelandic does.

Here að is the infinitive marker, like English to in to put.

So:

• setja = put
• að setja = to put

After nýbúin, Icelandic normally uses að + infinitive:

• nýbúin að borða = just finished eating
• nýbúin að lesa = just finished reading
• nýbúin að setja = just finished putting

So að is not a separate preposition here; it is just part of the infinitive construction.

Setja usually means to put, to place, or to set.

In this sentence:

• setja mold í beðið = put soil into the bed

It is a very natural verb when talking about placing something somewhere.

A learner may notice that English sometimes says fill the bed with soil or put soil in the bed. Icelandic is using the straightforward put verb here.

In Icelandic, mold means soil, earth, or dirt.

So in this sentence it means garden soil, not fungus or mildew.

That can be confusing for English speakers, because English mold/mould usually refers to the fuzzy growth on old food or damp walls. Icelandic mold is a normal word for earth/soil.

Here beðið means the bed, specifically a garden bed, flower bed, or planting bed.

The basic noun is:

• beð = bed

And beðið is the definite singular form:

• beð = a bed
• beðið = the bed

Because the sentence is about putting soil into it, this is clearly a garden bed, not a sleeping bed.

This is about case after the preposition í.

The preposition í can take:

• accusative when there is movement into something
• dative when something is located in something

Here the meaning is put soil into the bed, so there is motion/change of location. That is why Icelandic uses the accusative:

• í beðið = into the bed

If you were describing location instead, you would use dative:

• Moldin er í beðinu. = The soil is in the bed.

So:

• setja mold í beðið = put soil into the bed
• vera í beðinu = be in the bed

It is both.

The noun beð is a neuter noun, and in the singular its nominative and accusative forms are the same. So:

• beðið can be nominative definite singular
• beðið can also be accusative definite singular

In this sentence, it is functioning as accusative because í takes the accusative when meaning into.

The dative definite form would be different:

• beðinu

That difference helps show the grammar clearly.

You could translate it that way to understand the structure, but it would not sound natural in English.

A better natural English translation is:

• I’ve just put soil in the bed.
• I’ve just put soil into the garden bed.
• I’ve just finished putting soil in the bed.

So the literal structure is useful for learning the grammar, but not for producing natural English.

Usually, yes. Vera nýbúinn að + infinitive normally implies that the action has just been completed.

So Ég er nýbúin að setja mold í beðið suggests:

• the speaker finished putting the soil in very recently
• the action is now done

It is not just I was putting soil in the bed; it is more like I’ve just done that.

Yes. The main part that changes is nýbúinn / nýbúin / nýbúið, because it agrees with the subject.

For example:

• Ég er nýbúinn að setja mold í beðið. — male speaker
• Ég er nýbúin að setja mold í beðið. — female speaker

If the subject changes, the form changes too:

• Við erum nýbúin að setja mold í beðið. — we have just put soil in the bed
  
  • this form is often used for a mixed group or a feminine group, depending on context

So yes, this expression is very sensitive to agreement.

Yes, very common.

Icelandic often uses vera nýbúinn að + infinitive where English would use:

• have just done
• just did
• have just finished doing

Examples:

• Ég er nýbúin að borða. — I’ve just eaten.
• Hún er nýbúin að hringja. — She has just called.
• Við erum nýbúin að klára þetta. — We’ve just finished this.

So this sentence is a very natural everyday pattern, not an unusual one.