Сегодня я съел одну вафлю с клубничным джемом, а вторую — без него.

Because съел is the perfective past tense of съесть (to eat up / to eat completely), while ел is the imperfective past tense of есть (to eat).

In this sentence, the speaker is talking about two completed actions:

• he ate one waffle with jam
• and the second one without it

So Russian naturally uses the perfective:

• я съел = I ate / I finished eating

If you used ел, it would sound more like:

• I was eating
• I used to eat
• or it would focus on the process rather than the completed result

So here съел fits because the action is presented as finished.

Yes. In the past tense, Russian verbs agree with gender and number.

• съел = masculine singular
• съела = feminine singular
• съело = neuter singular
• съели = plural

So я съел strongly suggests that the speaker is male.

If the speaker were female, the sentence would begin:

• Сегодня я съела...

Because вафлю is the accusative singular form of вафля, and одну is the matching accusative singular feminine form of один.

The verb съесть takes a direct object, and direct objects are often in the accusative case.

So:

• nominative: одна вафля = one waffle as the subject
• accusative: одну вафлю = one waffle as the object

Here the waffle is what was eaten, so accusative is required.

Forms:

• одна → одну
• вафля → вафлю

No. вторую does not mean two.

It means the second (one).

This is the accusative singular feminine form of второй (second), and it matches the implied noun вафлю.

So:

• одну вафлю = one waffle
• вторую = the second one

In fuller form, you could say:

• а вторую вафлю — без него

But Russian often omits repeated nouns when they are obvious from context.

Because the sentence is specifically contrasting the first of two waffles with the second of the two.

• вторую = the second one
• другую = another / a different one

Here the idea is clearly:

• one waffle had strawberry jam
• the second waffle did not

So вторую is the most precise choice.

Because the preposition с in the meaning with takes the instrumental case.

So both the adjective and the noun go into instrumental:

• клубничный джем = nominative
• с клубничным джемом = instrumental after с

Forms:

• клубничный → клубничным
• джем → джемом

This is a very common pattern:

• с чаем = with tea
• с молоком = with milk
• с клубничным джемом = with strawberry jam

Него refers to джем.

The second half means:

• and the second one without it

Since джем is masculine, the pronoun used is masculine too.

Also, the preposition без (without) requires the genitive case, so:

• nominative: он
• genitive after без: него

So:

• без него = without it

Here it means the jam.

After certain prepositions, the third-person pronouns often take an initial н-.

So after без, you say:

• без него = without him/it
• без неё = without her/it
• без них = without them

Not:

• без его
• без её
• без их

This н- is a normal grammatical feature of Russian pronouns after prepositions.

In this sentence, него refers to джем, not вафлю.

That is because the phrase before it is:

• с клубничным джемом = with strawberry jam

Then:

• без него = without it

So the contrast is:

• one waffle with jam
• the second waffle without jam

Since джем is masculine, the pronoun is masculine:

• него

If it referred to вафля, you would expect a feminine pronoun:

• без неё

But that would not make sense here, because the speaker ate the second waffle, not a second waffle without the waffle.

The dash marks an omission: some words are left out because they are easy to understand from context.

The full idea is something like:

• а вторую вафлю я съел без него
• or simply а вторую вафлю — без него

Russian often leaves out repeated words when they are obvious, and the dash helps show that something has been omitted.

So the dash here is a punctuation signal that the sentence is shortened, but the meaning is still clear:

• the second one — without it

А is often used to show contrast or comparison between two parts of a sentence.

Here the speaker is comparing two waffles:

• one was with strawberry jam
• the second was without it

So а works very naturally:

• ..., а вторую — без него
• ..., while/as for the second one — without it

Compare:

• и = and, simple addition
• но = but, stronger contradiction
• а = contrast, switching to a different but related point

This sentence is not really a strong but-type contradiction, so а is the most natural choice.

Yes, Russian word order is flexible.

Сегодня is placed first because it sets the time frame right away:

• Today, I ate...

That is very natural in Russian.

You could also say:

• Я сегодня съел одну вафлю...

Both are correct, but the emphasis changes slightly:

• Сегодня я съел... puts today in a more prominent position
• Я сегодня съел... starts with I

Russian often moves words around for emphasis, topic, and rhythm.

Because Russian does not need to repeat everything if the meaning is already clear.

The first half says:

• одну вафлю = one waffle

Then the second half says:

• вторую = the second one

The noun вафлю is understood, so it is omitted.

English does something similar:

• I ate one waffle with jam, and the second without it.

You do not have to repeat waffle there either.

Yes. Russian often shortens repeated material, but a fuller version could be:

• Сегодня я съел одну вафлю с клубничным джемом, а вторую вафлю съел без него.

This is grammatical, but more repetitive.

The original version is more natural because Russian prefers to avoid unnecessary repetition when the context is clear:

• а вторую — без него

That sounds smoother and more idiomatic.