Learn Hardcore Russian: Доктор сказал, что острое и горькое нельзя есть вечером. - The doctor said that spicy and bitter food must not be eaten in the evening.

So нельзя есть вечером is best understood as you must not eat in the evening (or one should not eat in the evening), not you are physically unable to eat. Context decides whether нельзя is prohibition or impossibility; here it is clearly prohibition or a medical recommendation.

Why are острое and горькое in the neuter form with -ое endings? Where is the noun?

острое and горькое are adjectives used as nouns. The full phrase would be something like:

острая еда = spicy food
горькая еда = bitter food

Russian often drops a generic noun like еда (food) and uses the adjective alone in the neuter singular to mean everything that is X:

острое ≈ spicy things / spicy food
горькое ≈ bitter things / bitter food

So острое и горькое нельзя есть = you must not eat spicy and bitter (things/foods).

Could we say острую и горькую еду instead of острое и горькое?

Yes, you can say:

Доктор сказал, что острую и горькую еду нельзя есть вечером.

This is fully correct and a bit more explicit: spicy and bitter food.

Using острое и горькое feels a bit more general and colloquial, like anything spicy or bitter, and it sounds very natural in everyday speech. Both versions are fine.

Why is есть used here instead of кушать? What's the difference?

есть is the basic, neutral verb for to eat. It is used in all contexts, including formal speech, writing, and grammar examples.

кушать is more polite / child-directed / soft. Adults often say кушать to children or in polite speech to guests.

In a neutral sentence like this, есть is the default choice. You could say нельзя кушать вечером, but it would sound a bit softer, and with a doctor it might feel slightly less formal.

Why is вечером used without a preposition, and why is it in the instrumental case?

вечером is the instrumental singular of вечер.

Russian often uses the instrumental case without a preposition to express time when:

утром = in the morning
днём = in the daytime
вечером = in the evening
ночью = at night

So нельзя есть вечером = you must not eat in the evening. This is just a fixed way of expressing time in Russian.

Why is there no explicit subject like "you" in нельзя есть вечером?

The construction нельзя + infinitive is impersonal. It does not say who exactly must not do the action.

It has a general meaning, like English one must not eat in the evening or you (in general) must not eat in the evening.

If you want to specify the person, you can add a dative:

Мне нельзя есть вечером. = I must not eat in the evening.
Тебе нельзя есть вечером. = You must not eat in the evening.

What tense and aspect is сказал? Why not говорил?

сказал is:

past tense, masculine form
of the perfective verb сказать (to say, to tell as a completed act).

Using сказал shows a single, completed act: the doctor said it (once, as a specific statement).

говорил is imperfective (говорить = to speak, to be saying). Доктор говорил, что… would emphasize an ongoing or repeated action in the past: the doctor was saying / used to say / would say that….

Why do we need что after Доктор сказал? Can it be omitted like "The doctor said you must not eat..." in English?

In Russian, after verbs like сказать, думать, знать, etc., the conjunction что introducing a clause is almost always required:

Доктор сказал, что нельзя есть вечером.

Omitting что is possible only in very limited, colloquial or special styles; in normal speech and writing you should keep что here. So unlike English, you should assume что is needed.

Can we change the word order and say: Доктор сказал, что вечером нельзя есть острое и горькое?

Yes, that is perfectly correct:

Доктор сказал, что вечером нельзя есть острое и горькое.

Both orders are grammatical:

…что острое и горькое нельзя есть вечером.
…что вечером нельзя есть острое и горькое.

They mean the same thing. The difference is just in emphasis: in version 1 the focus is more on what you must not eat; in version 2 a bit more on when you must not eat it.

Why is доктор masculine here? What if the doctor is a woman?

The noun доктор is grammatically masculine in Russian, regardless of the actual gender of the person. So you normally use masculine forms:

Доктор сказал… (even if the doctor is female, in neutral style).

In colloquial speech some people might say доктор сказала if they clearly mean a female doctor, but the standard, especially in formal style, is masculine agreement with доктор.

Does острое mean "spicy" or "hot" or "sharp"?

The adjective острый can mean several things depending on context:

острый нож = a sharp knife
острый нос = a sharp (pointed) nose
острая еда = spicy food, hot (because of chili, pepper, etc.)

In the sentence острое нельзя есть, we are talking about taste, so острое = spicy things / spicy food (not sharp objects). Context rules out the sharp meaning.

Does горькое always mean "bitter food"? I heard it can mean alcohol.

горькое literally means bitter (things), from the adjective горький.

In context of food and drink it can mean:

bitter food (like very bitter vegetables, some medications)
bitter drinks, especially strong alcohol (vodka and similar) in colloquial speech

So горькое нельзя есть вечером can include bitter foods and, depending on context, possibly strong alcohol as well. Without more context, it is safest to translate it as bitter (or very strong / harsh-tasting) food and drink.

Why are острое and горькое singular and neuter, not plural like острые и горькие?

When adjectives are used as nouns in Russian to mean a whole class of things, they very often appear in the neuter singular:

сладкое = sweets / dessert
солёное = salty things / pickles
жареное = fried things
острое = spicy things
горькое = bitter things

This neuter singular form expresses the idea everything that is X. So острые и горькие would just be adjectives describing some specific plural nouns, but острое и горькое function as abstract classes of food.

Is есть an infinitive here, and why is that important?

Yes, есть in this sentence is in the infinitive form.

The pattern нельзя + infinitive is a fixed impersonal construction meaning it is not allowed / one must not do X. Some other examples:

Нельзя курить. = You must not smoke.
Нельзя опаздывать. = You must not be late.

So the infinitive есть is exactly what you expect after нельзя.

Is there any difference in meaning between нельзя есть вечером and нельзя вечером есть?

Both word orders are grammatically correct:

нельзя есть вечером
нельзя вечером есть

They mean the same: you must not eat in the evening.

In everyday speech, нельзя есть вечером is slightly more common, but нельзя вечером есть is also perfectly natural. Russian word order is flexible; both emphasize the prohibition rather than making a strong contrast.

How would the sentence change if we wanted to say "The doctor said that I must not eat spicy and bitter things in the evening"?

You would add мне in the dative to show who the prohibition applies to:

Доктор сказал, что мне нельзя есть острое и горькое вечером.

Now it is clear that I (not people in general) must not eat spicy and bitter things in the evening. The verb form нельзя stays the same; you only add the dative pronoun.