Рубеж метания находится на полу.

Because метания is in the genitive singular.

In Russian, one noun often modifies another by using the genitive case. Here:

• рубеж = boundary / line / limit
• метание = throwing
• рубеж метания = literally the boundary/line of throwing

So метание changes to метания to show that relationship.

A rough English parallel would be something like throwing line, but Russian expresses it as line of throwing.

Here it is singular: genitive singular of метание.

This is a very common point of confusion because for nouns ending in -ие, the form -ия can be:

• genitive singular: метания
• or nominative/accusative plural: also метания

So the spelling is the same, but the grammar tells you which one it is. In рубеж метания, it is genitive singular because it depends on рубеж.

Рубеж is in the nominative singular.

It is the subject of the sentence:

• Рубеж метания = the throwing line
• находится = is located
• на полу = on the floor

Also, рубеж is a masculine noun.

Because находится means is located / is situated, which fits this sentence well.

In Russian:

• есть is usually not used like English is in ordinary present-tense sentences.
• Russian often simply leaves out the present-tense verb to be.
• But when you want to stress location, находится is very common.

So:

• Рубеж метания находится на полу. = The throwing line is located on the floor.

A more conversational sentence might sometimes omit the verb, depending on context, but находится sounds clear and natural, especially in formal, technical, or instructional language.

Because this sentence describes location, not movement.

With на, Russian usually works like this:

• на + accusative = motion onto something
• на + prepositional = location on something

So:

• на пол = onto the floor
• на полу = on the floor

Here the line is already there, so Russian uses the prepositional idea of location: на полу.

Compare:

• Положить мяч на пол. = Put the ball onto the floor.
• Мяч лежит на полу. = The ball is on the floor.

Because пол has a special location form: на полу.

A small group of masculine nouns use a special locative/prepositional form in -у / -ю after в or на when talking about physical location.

Examples:

• на полу = on the floor
• в лесу = in the forest
• в шкафу = in the cupboard

So на полу is the standard expression.

Because Russian has no articles like a or the.

Russian usually leaves definiteness to context. So:

• Рубеж метания находится на полу.

can be understood as:

• The throwing line is on the floor
• or sometimes A throwing line is on the floor

In most real contexts, the speaker and listener already know which line is meant, so English naturally uses the.

No, Russian word order is fairly flexible.

The most neutral order here is:

• Рубеж метания находится на полу.

But you could also say:

• На полу находится рубеж метания.

That version puts more emphasis on where it is.

Even though word order can change, the case endings still show the grammatical relationships, so Russian does not rely on word order as heavily as English does.

The stress is:

• рубЕ́ж метА́ния нахО́дится на полу́

More standard spelling with stress marks:

• рубеж́ is incorrect visually, so better written as рубеж with stress remembered as рубеж
• full stress pattern: рубе́ж мета́ния нахо́дится на полу́

A rough English-style approximation would be:

• roo-BYEZH meh-TAH-nee-ya nah-KHO-deet-sya na pa-LOO

The main thing to remember is the final stress in полу́.

Рубеж literally means something like boundary, limit, or line.

Depending on context, it can refer to:

• a border or frontier
• a limit or boundary
• a marked line in sports, safety instructions, or technical descriptions

So in this sentence, рубеж метания means the marked line or boundary connected with throwing — in natural English, something like the throwing line or the throw line.