На полу лежали гантели, а у стены стояла штанга.

Because Russian makes a basic distinction between location and movement.

• на пол = onto the floor (movement toward the floor)
• на полу = on the floor (location)

In this sentence, the dumbbells are already there, so Russian uses the location form: на полу.

полу is the form used after на to mean on the floor. It is one of those nouns that has a special location form, often called the locative or second prepositional.

So learners usually just memorize:

• пол = floor
• на полу = on the floor

It may look irregular, but it is completely standard Russian.

Here у means by, next to, or near.

It takes the genitive case, so:

• стена → стены

So у стены means by the wall or near the wall.

It does not mean on the wall or in the wall. It means the barbell was positioned beside the wall.

Russian often uses position verbs where English might just use was/were.

• лежать = to lie, to be lying
• стоять = to stand, to be standing

So Russian describes not only that the objects existed there, but also how they were positioned.

• гантели лежали = the dumbbells were lying
• штанга стояла = the barbell was standing

This is very natural in Russian. It makes the picture more specific.

Usually, yes, or at least it suggests that it was in an upright or propped-up position, probably leaning against the wall.

If the barbell had been flat on the floor, Russian would more likely use лежала instead.

So the verb choice gives you a visual clue about the scene.

гантели here is plural: dumbbells.

The singular is гантель: dumbbell.

So:

• гантель лежала = one dumbbell was lying
• гантели лежали = dumbbells were lying

Because Russian past tense agrees with the subject.

• гантели is plural, so the verb is лежали
• штанга is singular feminine, so the verb is стояла

In the past tense:

• plural → -ли
• feminine singular → -ла

So the verb endings match the nouns.

That word order is very natural when Russian is describing a scene.

The pattern is often:

place → verb → thing

So:

• На полу лежали гантели
• У стены стояла штанга

This is similar to English On the floor lay dumbbells. It sounds descriptive and scene-setting.

You could also say:

• Гантели лежали на полу
• Штанга стояла у стены

That is also correct, but it gives a slightly different focus, with more attention on the objects themselves.

а often means and, but with a slight sense of contrast, comparison, or meanwhile.

So here it connects two parts of the scene:

• the dumbbells were on the floor
• the barbell was by the wall

Using а makes the sentence feel like one thing was here, while another thing was there.

If you used и, it would sound more like a simple addition. А is more natural here because the sentence is comparing two positions.

Yes, that would be grammatical.

But it is less specific. были / была would only mean that the objects were there.

The original sentence is more natural and vivid because it uses position verbs:

• лежали tells you the dumbbells were lying
• стояла tells you the barbell was standing

Russian often prefers this more visual way of describing where things are.