Learn Hardcore Czech: Můj bratr má rád ovoce a říká, že je to jeho nejzdravější jídlo. - My brother likes fruit and says that it is his healthiest food.

bratr (brother) is masculine animate.
For masculine singular nouns in the nominative, you use můj.
moje is used for feminine and neuter nouns in the nominative singular (e.g. moje sestra, moje auto).

So:

Můj bratr = my brother
Moje sestra = my sister

What exactly does má rád mean, and why not just a single verb for “likes”?

Czech often uses the phrase mít rád (literally “to have glad”) to mean “to like (someone/something)”.

mít = to have
rád = glad/pleased (short-form adjective)

So:

Můj bratr má rád ovoce. = My brother likes fruit.

There is no single everyday verb that works exactly like English “like” for objects and people; mít rád is the normal neutral choice.

Other related verbs:

líbit se – to be pleasing (used mostly for things you find nice/pleasant/attractive, often visually)
- Líbí se mi to. = I like it / It appeals to me.
milovat – to love (stronger emotion)
- Miluju ovoce. = I love fruit (very strong liking).

Could I say Rád mám ovoce instead of Mám rád ovoce?

Grammatically it’s possible, but it sounds unusual or stylistically marked.

The normal, idiomatic order is:

Mám rád ovoce.

Putting rád before mám (e.g. Rád mám ovoce) is not completely wrong, but it would sound odd in everyday speech and might feel like you are emphasizing rád in a strange way. Learners should stick to the fixed phrase mít rád + [accusative] with the usual order:

mám rád
máš rád
má rád
etc.

Why is it ovoce with no visible ending change? Is ovoce singular or plural?

Ovoce is a neuter, usually uncountable (mass) noun meaning “fruit” in general.

In the nominative singular, it is ovoce.
In the accusative singular, it is also ovoce.

So in má rád ovoce, ovoce is in the accusative singular, but its form happens to be the same as the nominative.

In practice:

Ovoce je zdravé. = Fruit is healthy. (verb in singular)
Mám rád ovoce. = I like fruit.

Czech normally doesn’t use a plural for “fruits” the way English does. Instead of a plural like “ovoce” → “ovocesomething”, speakers say:

druhy ovoce = kinds of fruit
různé druhy ovoce = different kinds of fruit

Why is there a comma before že in říká, že je to …?

In Czech, you normally put a comma before a conjunction that introduces a subordinate clause.

že introduces an object clause (reported speech/thought), so a comma is required:

říká, že… = he says that …

Other conjunctions that usually take a comma before them:

protože (because) – Odešel, protože byl unavený.
když (when) – Když přijde, zavolám.

What does že mean here, and how is it different from protože?

In this sentence, že means “that” in the sense of reported speech:

říká, že je to jeho nejzdravější jídlo
= he says that it is his healthiest food

protože means “because” and introduces a reason:

Ovoce jí často, protože je zdravé.
= He eats fruit often because it is healthy.

So:

že → “that” (for what someone says, thinks, believes, etc.)
protože → “because” (gives the reason)

In že je to jeho nejzdravější jídlo, what does to refer to, and why is it needed?

to is a neuter pronoun that stands for “that / it”, referring back to ovoce.

Czech often uses to in sentences with the verb být (to be) when saying “X is Y” in a more neutral or explanatory way:

Ovoce je zdravé. = Fruit is healthy. (simple statement)
Ovoce je to nejzdravější jídlo. = Fruit is the healthiest food.

In your sentence, inside the že-clause:

že je to jeho nejzdravější jídlo
= that it is his healthiest food

The to makes the sentence feel natural and complete.
You could technically say že je jeho nejzdravější jídlo, but in practice že je to jeho… is much more idiomatic.

Could I say že to je jeho nejzdravější jídlo instead of že je to jeho nejzdravější jídlo?

Yes, both are grammatically correct:

říká, že je to jeho nejzdravější jídlo
říká, že to je jeho nejzdravější jídlo

The difference is very small and mostly about rhythm and slight emphasis.

že je to jeho… is the more common and neutral word order in this kind of clause.
že to je jeho… can put a tiny bit more emphasis on to (“that it is that which is his healthiest food”), but in everyday speech most people would not feel a strong difference.

For learners, že je to jeho… is the safest, most natural choice.

How is nejzdravější formed, and what does it literally mean?

nejzdravější is the superlative of the adjective zdravý (healthy).

Formation:

Base adjective: zdravý = healthy
Comparative: zdravější = healthier
- add -ější to the stem
Superlative: nejzdravější = the healthiest
- add the prefix nej- to the comparative form

So literally:

zdravý → healthy
zdravější → more healthy / healthier
nejzdravější → most healthy / the healthiest

Why does nejzdravější come before jídlo and not after, like in English “food healthiest”?

In Czech, adjectives normally come before the noun they modify:

zdravé jídlo = healthy food
nejzdravější jídlo = the healthiest food

Putting the adjective after the noun (e.g. jídlo nejzdravější) is possible only in special, more poetic or stylistic contexts. For normal speech and writing, you should always put adjectives like zdravý / nejzdravější before the noun.

What exactly does jídlo mean here, and could I use potrava or strava instead?

jídlo is the most common everyday word and here means “food (as something you eat)”. It can also mean “a meal” depending on context.

nejzdravější jídlo = the healthiest food (item/type of food)

potrava and strava are more formal or technical:

potrava – nourishment/food in a biological sense (used for animals, nutrition science, etc.)
strava – diet, food intake, often in health/medical contexts (zdravá strava = healthy diet)

In this casual sentence about a brother, jídlo is the most natural and typical word.