Tato omáčka je lepší, když do ní krájím rajčata a cibuli.

Both are possible, but they feel a little different.

• tato omáčka = this sauce in a more neutral or careful way
• ta omáčka = also this sauce / that sauce, but more colloquial in everyday speech

Here, tato is the standard demonstrative form for a feminine singular noun.

Because omáčka is feminine singular, this has to match it:

• masculine: ten
• feminine: ta / tato
• neuter: to / toto

So tato omáčka is simply the grammatically matching form for this sauce.

Because lepší is the correct comparative form of dobrý (good) in Czech.

The pattern is:

• dobrý = good
• lepší = better
• nejlepší = best

So:

• Ta omáčka je dobrá. = The sauce is good.
• Tato omáčka je lepší. = This sauce is better.

You do not build this comparative by just changing the ending of dobrá. It is an irregular comparative, similar to English good → better.

Also, lepší does not change for gender here:

• ten film je lepší
• ta omáčka je lepší
• to víno je lepší

In Czech, subordinate clauses are normally separated by a comma.

Here:

• main clause: Tato omáčka je lepší
• subordinate clause: když do ní krájím rajčata a cibuli

So the comma is required.

This is very common in Czech:

• Vím, že přijde.
• Řeknu ti, až budu doma.
• Je to lepší, když...

English sometimes uses fewer commas, but Czech is stricter about them.

Here když is best understood as when in the sense of whenever.

So the sentence means something like:

• This sauce is better when I cut tomatoes and onion into it.
• or more naturally, This sauce tastes better if/when I add sliced tomatoes and onion to it.

In Czech, když often introduces:

• a specific time: Když přišel, začalo pršet. = When he arrived, it started raining.
• a general repeated situation: Je to lepší, když... = It’s better when/if...

If you wanted a more purely conditional if, Czech often uses jestli or pokud, but když is very natural in this kind of general statement.

Because the verb phrase here expresses movement into the sauce.

• do = into
• v = in, inside

So:

• do ní krájím rajčata = I cut tomatoes into it
• v ní would mean in it, describing location rather than direction

Compare:

• Rajčata jsou v ní. = The tomatoes are in it.
• Krájím rajčata do ní. = I am cutting the tomatoes into it.

Also, Czech prepositions require specific cases. Do takes the genitive, which is why the pronoun appears as ní here.

Because the preposition do requires a special case form.

The noun being referred to is omáčka, which is feminine. The pronoun it/her for a feminine noun changes depending on its grammatical role.

Relevant forms:

• ji = accusative direct object
• ní = genitive/locative/instrumental form used after some prepositions

Since do takes the genitive, you get:

• do ní = into it

Examples:

• Vidím ji. = I see it/her.
• Dám to do ní. = I put it into it.

So ji would be wrong after do.

Because the sentence describes a general habit or repeated situation.

• krájím = I cut / I am cutting

In Czech, the present tense is often used for things that are generally true or regularly done:

• Káva chutná lépe, když přidávám mléko.
• Polévka je lepší, když do ní dávám česnek.

So když do ní krájím rajčata a cibuli means:

• when I cut tomatoes and onion into it
• whenever I do that

It does not have to mean that you are literally doing it right now. It can describe your usual method.

Because the sentence focuses on the process or repeated action, not on one completed result.

krájet is imperfective:

• krájím = I cut / I am cutting / I cut habitually

A perfective verb would suggest a completed one-time action, which is less natural in this general statement.

This sentence is about a regular cooking practice:

• The sauce is better when I cut tomatoes and onion into it.

That is why the imperfective verb fits well.

A perfective alternative might appear in a different type of sentence, for example if you were talking about one completed event.

Rajčata is the accusative plural form of rajče (tomato).

Here, rajčata is the direct object of krájím, so it must be in the accusative.

For neuter nouns like rajče, the nominative plural and accusative plural are often the same:

• nominative plural: rajčata
• accusative plural: rajčata

So in this sentence:

• krájím rajčata = I cut tomatoes

That is why it looks the same as the dictionary-style plural form.

Because cibuli is the accusative singular of cibule (onion).

The verb krájím takes a direct object, so cibule must change into the accusative:

• nominative: cibule
• accusative: cibuli

So:

• cibule je na stole = the onion is on the table
• krájím cibuli = I am cutting the onion

In the sentence, you have:

• rajčata = tomatoes, plural
• cibuli = onion, singular

That combination is perfectly normal: tomatoes and onion.

Because Czech, like English, often uses whatever quantity sounds natural for the ingredient.

So this can mean something like:

• tomatoes and onion
• some tomatoes and an onion

That is not unusual in recipe language.

You could also have other combinations:

• rajče a cibuli = a tomato and an onion
• rajčata a cibule would not work here, because cibule would need accusative, not nominative
• rajčata a cibule could work only in a different grammatical role

So the singular/plural difference is about meaning, not about grammar agreement between the two nouns.

Yes, it is natural in Czech.

krájet něco do něčeho means:

• to cut something into something
• to slice something directly into a dish, bowl, soup, sauce, etc.

So:

• krájím rajčata do salátu = I cut tomatoes into the salad
• krájím cibuli do polévky = I cut onion into the soup
• krájím rajčata a cibuli do ní = I cut tomatoes and onion into it

In English, you might translate it more naturally as:

• I add chopped tomatoes and onion to it
• I slice tomatoes and onion into it

So the Czech is idiomatic even if the most natural English wording may be slightly different.

Yes. Czech word order is flexible, although some versions sound more neutral than others.

The given sentence:

• Tato omáčka je lepší, když do ní krájím rajčata a cibuli.

is natural and neutral.

You could also hear:

• Tato omáčka je lepší, když krájím do ní rajčata a cibuli.
• Když do ní krájím rajčata a cibuli, tato omáčka je lepší.

But the original version is probably the best everyday choice.

Czech word order often changes to show:

• topic vs. new information
• emphasis
• rhythm

So the sentence can be rearranged, but not all versions feel equally natural.

Literally, it means better, but in context it often implies tastes better.

With food, Czech often says:

• je lepší = is better when English might naturally say:
• tastes better

So this sentence can be understood as:

• This sauce is better when I cut tomatoes and onion into it.
• more natural English: This sauce tastes better when I add sliced tomatoes and onion.

The Czech itself does not explicitly say chutná lépe (tastes better), but that meaning is easy to infer from context.