Границу участка измерили утром.

Because границу is the accusative singular form of граница.

• Dictionary form: граница = boundary, border
• In this sentence, it is the direct object of измерили (measured)
• Feminine nouns in -а usually change to -у in the accusative singular

So:

• граница = the boundary (subject form / nominative)
• границу = the boundary (object form / accusative)

Here, the boundary is the thing that was measured, so Russian uses границу.

Because участка is the genitive singular of участок, and it depends on граница.

Russian often expresses of relationships with the genitive case:

• граница участка = the boundary of the plot
• literally: boundary + of the plot

So:

• участок = plot, parcel of land, site
• участка = of the plot

This is a very common pattern in Russian:

• крыша дома = the roof of the house
• дверь комнаты = the door of the room
• граница участка = the boundary of the plot

This is a very common Russian construction called an indefinite-personal sentence.

Измерили is:

• past tense
• plural
• from измерить = to measure

When Russian uses a 3rd person plural past form with no subject, it often means:

• they measured
• someone measured
• people measured

The actual doer is not named because it is unknown, unimportant, or obvious from context.

So Границу участка измерили утром can mean:

• They measured the boundary of the plot in the morning
• or, in natural English, The boundary of the plot was measured in the morning

Russian often prefers this kind of active-looking sentence where English might use a passive.

Измерили is perfective.

That means the sentence presents the action as completed: the measuring was done and finished.

Compare:

• измерили = measured, finished measuring
• измеряли = were measuring / measured (without emphasizing completion), possibly as a process or repeated action

So this sentence is not just saying that measuring was going on in the morning. It says that the boundary got measured in the morning.

Because Russian often uses the instrumental case by itself for certain time expressions.

Here:

• утро = morning
• утром = in the morning

This is a fixed and very common adverbial form.

Similar examples:

• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So утром functions almost like an adverb here.

Russian word order is much more flexible than English word order.

All of these can be grammatical, but they can sound slightly different in emphasis.

In Границу участка измерили утром:

• границу участка comes first, so it is what the sentence is mainly about
• утром comes last, which can make the time feel like the new or important piece of information

Very roughly:

• Границу участка измерили утром = As for the boundary of the plot, it was measured in the morning.
• Утром измерили границу участка = In the morning, they measured the boundary of the plot.
• Измерили границу участка утром = They measured the boundary of the plot in the morning.

The basic meaning stays the same, but the focus shifts a little.

Yes. Very naturally.

Although the Russian sentence uses an active verb form (измерили = they measured), English will often translate it as a passive:

• The boundary of the plot was measured in the morning

That is because Russian commonly uses the indefinite-personal construction where English prefers a passive when the doer is unknown or unimportant.

A more explicitly passive Russian version would be:

• Граница участка была измерена утром.

That is also correct, but it can sound more formal or more written. The original sentence is very normal and natural.

The structure is:

Direct object + dependent genitive noun + verb + time expression

Breaking it down:

• Границу = the boundary (direct object, accusative)
• участка = of the plot (genitive)
• измерили = they measured / was measured
• утром = in the morning

So literally:

• Boundary of-the-plot measured in-the-morning

A more natural English rendering is:

• They measured the boundary of the plot in the morning
• or The boundary of the plot was measured in the morning

Участок has several meanings depending on context, but in this sentence, with граница, it most naturally means a plot of land, parcel, or lot.

Common meanings of участок include:

• a land plot
• a section or stretch of something
• an area
• a district/precinct in some contexts

Because the sentence talks about граница участка (the boundary of the plot), the land-related meaning is the most likely one.