Min nabo låner os sin plæneklipper, når græsset bliver for højt, og der ligger ofte en snegl i græsset foran den.

In Danish, låne can mean both lend and borrow. The context tells you which one is meant.

Here, min nabo is the subject and os is the person receiving the item, so the meaning is My neighbor lends us his/her lawn mower.

Compare:

• Min nabo låner os sin plæneklipper = My neighbor lends us his/her lawn mower
• Vi låner en plæneklipper af naboen = We borrow a lawn mower from the neighbor

So Danish uses one verb where English usually uses two.

Os is the indirect object, meaning to us.

Danish can structure this just like English:

• låne nogen noget = lend someone something

So:

• Min nabo låner os sin plæneklipper

literally follows the pattern:

• My neighbor lends us his/her lawn mower

You do not need a preposition here.

Because sin is the reflexive possessive. It refers back to the subject of the clause.

Here, the subject is min nabo, so sin plæneklipper means his/her own lawn mower.

This is the key contrast:

• sin plæneklipper = the neighbor’s own lawn mower
• hans/hendes plæneklipper = someone else’s lawn mower

So Danish uses sin/sit/sine when the owner is the subject of the clause.

Because Danish normally writes compound nouns as a single word.

So:

• plæne = lawn
• klipper = cutter / mower

together become:

• plæneklipper = lawn mower

This is very typical in Danish. English sometimes writes similar words as two words, one word, or with a hyphen, but Danish strongly prefers one compound word.

Når is used for something that happens when or whenever a situation occurs, especially if it is expected or repeated.

Here the idea is:

• whenever the grass gets too high, the neighbor lends the mower

So når fits well.

By contrast, hvis is more like if, for a more open or uncertain condition.

A rough contrast:

• Når græsset bliver for højt = When/Whenever the grass gets too high
• Hvis græsset bliver for højt = If the grass gets too high

In this sentence, når sounds more natural because it describes a regular situation.

Because bliver means becomes / gets.

The sentence is describing a change of state: the grass was not too high before, and then it grows and gets too high.

So:

• græsset bliver for højt = the grass gets too high

If you used er, that would describe a state rather than the change into that state.

Because græsset is a neuter noun, and the adjective agrees with it.

The basic adjective is:

• høj = high / tall

With a neuter singular noun, it becomes:

• højt

So:

• græsset bliver for højt

If the noun were common gender singular, you would normally get høj instead.

This is adjective agreement in predicative position.

This is the difference between definite and indefinite reference.

• græsset = the grass
• en snegl = a snail

The grass is understood as the specific grass already being talked about, so it is definite.

The snail is being introduced as one example of something often found there, so it is indefinite.

Also notice how Danish usually marks definiteness at the end of the noun:

• græs → græsset
• snegl → sneglen

Here der is an existential there, like English there is.

So:

• der ligger ofte en snegl
  means something like
• there is often a snail

This der does not mainly mean location. It is a grammatical placeholder used to introduce the existence or presence of something.

So the structure is similar to:

• Der er en bog på bordet = There is a book on the table
• Der ligger ofte en snegl i græsset = There is often a snail lying in the grass

Danish often prefers a positional verb when describing where something is.

Common ones are:

• ligge = lie / be lying
• stå = stand / be standing
• sidde = sit / be sitting

So der ligger ofte en snegl i græsset is more specific and natural than just der er. It presents the snail as lying/resting there in the grass.

English often just says there is, but Danish often chooses a more physical location verb.

Because Danish main clauses normally follow the verb-second pattern.

In der ligger ofte en snegl, the first position is filled by der, so the finite verb ligger must come next.

After that comes the adverb ofte, and then the noun phrase en snegl.

So the order is:

• der
  • ligger
    • ofte
      • en snegl

This word order is very typical in Danish main clauses.

It most naturally refers to plæneklipper.

A very useful clue is gender:

• plæneklipper is common gender, so the pronoun is den
• græsset is neuter, so if it referred to the grass, you would expect det

So foran den means in front of it, with it referring to the lawn mower.

The sentence gives you several clues.

Common gender nouns:

• en nabo
• en plæneklipper
• en snegl

That is why you see forms like:

• min nabo
• sin plæneklipper
• den

Neuter noun:

• græs → græsset

That is why you get:

• the definite ending -et in græsset
• the adjective form højt

So the sentence is actually showing gender agreement in several places.

Yes. Danish allows that.

You could say:

• Når græsset bliver for højt, låner min nabo os sin plæneklipper, og der ligger ofte en snegl i græsset foran den.

When the subordinate clause comes first, the main clause still keeps the Danish verb-second rule, so låner comes before min nabo.

That is a very common Danish pattern:

• subordinate clause first
• then finite verb
• then subject

So word order changes, but the meaning stays basically the same.