Сырники вкуснее со сметаной, но сегодня мы ели их с вареньем.

Сырники is plural nominative here. The singular form is сырник.

So the sentence is talking about multiple pancakes/cakes, not just one. That is why the later pronoun is их = them.

Russian usually omits the present-tense form of to be.

So:

• Сырники вкуснее со сметаной
  literally: Syrniki tastier with sour cream
• natural English: Syrniki are tastier with sour cream

There is no separate word for are in this kind of present-tense sentence.

Вкуснее is the comparative form of вкусный = tasty.

So:

• вкусный = tasty
• вкуснее = tastier

The sentence is making a comparison: syrup pancakes are tastier with sour cream.

You can say более вкусные, but вкуснее is much more natural and common here.

A few differences:

• вкуснее = simple, natural comparative: tastier
• более вкусные = more analytical, sometimes more formal or heavier

In everyday Russian, Сырники вкуснее со сметаной sounds much better than Сырники более вкусные со сметаной.

No. Comparative forms like вкуснее do not change for gender, number, or case.

So the same form can be used with:

• сырник вкуснее = one syrnik is tastier
• сырники вкуснее = syrniki are tastier

That is different from normal adjectives like вкусный / вкусная / вкусные, which do agree.

Со is just a pronunciation variant of с.

Russian uses со before some consonant clusters when с would be awkward to pronounce. Since сметаной starts with см-, Russian prefers:

• со сметаной

But вареньем begins with a vowel sound, so plain с is easy:

• с вареньем

So the difference is mostly phonetic, not grammatical.

Both are in the instrumental case.

That is because с / со in the sense of with often takes the instrumental:

• со сметаной = with sour cream
• с вареньем = with jam/preserves

Here it expresses what the food is served or eaten with.

Because сметана is a feminine noun, and in the instrumental singular it becomes сметаной.

Basic pattern:

• nominative: сметана
• instrumental: сметаной

This is a very common feminine ending pattern.

Because варенье is a neuter singular noun, and after с it goes into the instrumental singular:

• nominative: варенье
• instrumental: вареньем

So the ending changes from -е to -ем.

Ели is the past tense plural of есть = to eat.

It matches the subject мы:

• я ел / ела = I ate
• мы ели = we ate

Since the sentence talks about what happened today, Russian uses the past tense: сегодня мы ели...

Ели is imperfective, while съели is perfective.

In this sentence, ели simply reports what they ate. It is neutral and natural.

If you said съели, the focus would be more on finishing them or eating them up completely.

So:

• мы ели их с вареньем = we ate them with jam
• мы съели их с вареньем = we ate them up with jam / finished them with jam

Both are possible in some contexts, but ели is less completion-focused.

Yes, it could be omitted.

Russian often drops subject pronouns when the verb already makes the subject clear:

• сегодня мы ели их с вареньем
• сегодня ели их с вареньем

Both can work.

Including мы can add a little clarity or emphasis, especially because the sentence contrasts a general statement with what we did today.

Их means them and refers back to сырники.

It is used because сырники is now the direct object of ели:

• мы ели их = we ate them

Russian often uses a pronoun to avoid repeating the noun:

• мы ели сырники с вареньем = also possible
• мы ели их с вареньем = more natural once сырники has already been mentioned

Both но and а can sometimes be translated as but, but they are not identical.

Here но shows a stronger contrast:

• general preference: сырники вкуснее со сметаной
• exception today: но сегодня мы ели их с вареньем

So но works well because the second part goes against the expectation created by the first part.

If you used а, the contrast would feel a bit softer or more like whereas.

Not completely. Russian word order is fairly flexible, but the original version is natural and neutral:

• Сырники вкуснее со сметаной, но сегодня мы ели их с вареньем.

You can move words for emphasis, for example:

• Сегодня мы ели их с вареньем = emphasizes today
• С вареньем мы ели их сегодня = emphasizes with jam

The case endings still show the grammatical relationships, so Russian can move words around more easily than English.

A useful stressed version is:

• Сы́рники вкусне́е со смета́ной, но сего́дня мы е́ли их с варе́ньем.

Main stresses:

• сы́рники
• вкусне́е
• смета́ной
• сего́дня
• е́ли
• варе́ньем

Getting the stress right is important, because Russian stress is not always predictable.