Okurka a rajče jsou v létě dobré do malého salátu.

Because each noun is singular by itself, but okurka a rajče means cucumber and tomato. Two singular nouns joined by a (and) make a plural subject, so Czech uses the plural verb jsou (are).

Compare English: A cucumber and a tomato are...

Czech has no articles like a or the, so context decides.

Here, the singular nouns can be understood generically, meaning the ingredients in general:
Cucumber and tomato are good in summer for a small salad.

In another context, it could also mean a cucumber and a tomato. In recipe-style or general statements, this kind of generic singular is very natural.

Dobré agrees with the whole subject okurka a rajče.

• okurka = feminine singular
• rajče = neuter singular

Together, they form a plural subject. Since there is no masculine animate noun in the subject, Czech uses the plural adjective ending -é:

• dobré = correct here

You would use dobří only with masculine animate plural subjects, such as people or male animals.

They are in the nominative, because they are the subject of the sentence.

You can tell because they are the things that are good:

• Okurka a rajče = the things being talked about
• jsou dobré = are good

So this is the normal subject form.

Because the preposition v here means in in a time expression, and with that meaning it takes the locative case.

• dictionary form: léto = summer
• locative singular: létě
• so: v létě = in summer

This is a very common Czech pattern:

• v zimě = in winter
• na jaře = in spring
• na podzim = in autumn

Literally, do malého salátu means into a small salad, but in natural English here it is better understood as:

• for a small salad
• good in a small salad
• suitable for a small salad

With food, Czech often uses dobrý do + genitive to mean that something is a good ingredient for a dish.

So jsou dobré do malého salátu means that cucumber and tomato work well as ingredients in a small salad.

Because the preposition do requires the genitive case.

The noun is:

• salát = salad

After do, it becomes:

• salátu = genitive singular

The adjective must match the noun in case, gender, and number:

• malý salát
• do malého salátu

So both words change because of do.

Yes, absolutely.

That version uses the literal plurals:

• okurky = cucumbers
• rajčata = tomatoes

Both versions are grammatical, but the nuance is slightly different:

• okurka a rajče can sound more generic, like talking about the ingredients as types of food
• okurky a rajčata sounds more like actual plural items

In everyday Czech, both kinds of wording are possible depending on context.

No. Czech word order is fairly flexible because the endings show the grammatical relationships.

The given order is natural:

Okurka a rajče jsou v létě dobré do malého salátu.

But you could also say:

V létě jsou okurka a rajče dobré do malého salátu.

That would put more emphasis on v létě (in summer).

So in Czech, changing word order often changes focus or emphasis, not the core meaning.

Because it gives the final piece of information: what they are good for.

A common Czech structure is:

subject + verb + time/place + adjective/complement + more specific phrase

So this sentence builds up naturally:

• Okurka a rajče = topic
• jsou = verb
• v létě = time
• dobré = evaluation
• do malého salátu = specific use

Putting do malého salátu at the end sounds natural because it completes the idea.