Один лепесток упал на стол.

Russian does not have articles, so nouns usually appear without a separate word for a or the.

That means лепесток can mean a petal or the petal, depending on context.

The word один does not normally function as an article. It literally means one and often adds the idea of one single petal. If you remove it, Лепесток упал на стол is still a normal sentence.

Because лепесток is a masculine singular noun.

The word один agrees with the noun in gender, number, and case:

• один for masculine
• одна for feminine
• одно for neuter
• одни for plural

So with лепесток, the correct form is один.

It is in the nominative singular.

Here, лепесток is the subject of the sentence, the thing that performed the action of falling. Subjects are normally in the nominative case in Russian.

Упал is the past tense masculine singular form of the verb упасть.

In Russian past tense, the verb agrees with the subject in gender and number:

• упал — masculine singular
• упала — feminine singular
• упало — neuter singular
• упали — plural

Because лепесток is masculine singular, the verb is упал.

This is an aspect question.

• упал is perfective
• падал is imperfective

Here, упал shows a single completed event: the petal fell.

By contrast, падал would suggest something like:

• was falling
• used to fall
• kept falling

So Один лепесток упал на стол presents the fall as one finished action.

Because Russian uses different cases after на depending on whether you mean movement or location.

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

So:

• на стол = onto the table
• на столе = on the table

In this sentence, the petal moved and landed there, so на стол is correct.

It actually is in the accusative, but for an inanimate masculine singular noun, the accusative looks exactly like the nominative.

So:

• nominative: стол
• accusative: стол

This is normal in Russian.

Compare that with an animate masculine noun, where the accusative is different:

• nominative: кот
• accusative: кота

So на стол is accusative, even though the form looks unchanged.

No, Russian word order is fairly flexible.

Один лепесток упал на стол is a neutral, natural order:
subject + verb + destination

But other orders are possible, for example:

• На стол упал один лепесток — puts more focus on where it fell
• Упал один лепесток на стол — more marked, sometimes more literary or expressive

The basic meaning stays the same, but the emphasis changes.

Yes, absolutely.

Without один, the sentence simply states that a petal fell onto the table.

With один, you usually get a stronger sense of:

• one petal
• a single petal
• sometimes a slight contrast, as in one petal fell...

So один is optional if you do not want to emphasize the number.

The stress is:

оди́н лепесто́к упа́л на стол

A useful thing to remember is that unstressed vowels in Russian are often reduced in pronunciation, especially о. So the written form is important, but the spoken form may sound less like a full o in unstressed positions.