Learn Hardcore French: Nous mangeons du riz nature ce soir.

Because du (de + le) is the partitive article used with mass nouns to mean an unspecified amount: some rice. Le riz would mean the rice (a specific rice you have in mind, or rice in general as a concept), and des riz is only used if you’re literally talking about multiple kinds or servings of rice (rare).

Je mange du riz. = I’m eating some rice.
J’aime le riz. = I like rice (in general).
Ils proposent des riz parfumés. = They offer different rices.

What exactly does nature mean here?

It means plain: without sauce, seasoning, butter, or other add-ins. In this food sense, nature is commonly treated as invariable (no gender/number change): du yaourt nature, des pâtes nature, du riz nature. You may also see the agreeing plural (yaourts natures) in some writing; both are accepted. You can also hear au naturel, which is a bit more formal/cookbook-style.

Why is the verb mangeons in the present if the action is this evening (ce soir)?

French often uses the present to talk about near-future plans when a time phrase is present. Nous mangeons du riz nature ce soir = We’re eating plain rice tonight. You can also use the near future for emphasis or clarity: Nous allons manger du riz nature ce soir. Both are natural.

Can I start the sentence with Ce soir?

Yes. Both word orders are fine:

Ce soir, nous mangeons du riz nature.
Nous mangeons du riz nature ce soir. A comma after a short fronted time phrase like Ce soir is common in writing.

How do I negate this?

Use ne … pas and change the partitive to de:

Nous ne mangeons pas de riz nature ce soir. With colloquial on:
On ne mange pas de riz nature ce soir.

What’s the difference between nous and on for we?

Nous is standard and more formal in speech.
On is what most people use in everyday conversation for we and takes third-person singular verbs. Examples:
Nous mangeons du riz nature ce soir.
On mange du riz nature ce soir.

Why is it spelled mangeons with an extra e?

Verbs ending in -ger insert a silent e before endings that start with o or a (like -ons) to keep the soft g sound (ʒ). So it’s nous mangeons, not mangons. Same pattern: nager → nous nageons, changer → nous changeons.

How do you conjugate manger in the present?

je mange
tu manges
il/elle/on mange
nous mangeons
vous mangez
ils/elles mangent (final -ent is silent)

How do you pronounce the whole sentence?

Approximate IPA: [nu mɑ̃ʒɔ̃ dy ʁi natyʁ sə swaʁ]
Tips:

nous [nu]
mangeons [mɑ̃ʒɔ̃] (nasal an [ɑ̃], soft g [ʒ], nasal on [ɔ̃])
du [dy] (rounded u)
riz [ʁi] (final z silent)
nature [natyʁ] (French u = [y], uvular r)
ce soir [sə swaʁ] No required liaisons here.

Is there any liaison in this sentence?

No obligatory liaisons:

nous mangeons: none (liaison after nous only before a vowel sound)
du riz: none
riz nature: none
nature ce: none

Is riz masculine or feminine, and does anything agree with it?

Riz is masculine, hence du (de + le). Nature in this food sense is commonly invariable, so you’ll see des pâtes nature, des yaourts nature. You may also encounter the plural natures; both are acceptable.

When do I use du/de la/de l’/des versus just de?

Positive, unspecified quantity of a mass/uncountable noun: du/de la/de l’/des
- Je mange du riz.
After negation or after a precise/quantifying expression, use de/d’:
- Je ne mange pas de riz.
- Beaucoup de riz, un bol de riz, trop de riz.
Before an adjective that precedes a plural noun, des often becomes de:
- De bonnes pâtes.

How would I replace du riz with a pronoun?

Use en to replace a noun introduced by de/du/de la/des:

Nous en mangeons ce soir. = We’re eating some (of it) tonight.

Can I say Nous mangeons le riz nature ce soir?

That sounds like you’re referring to a specific, identifiable rice (the plain rice, as opposed to another dish). For a general dinner plan, use the partitive: Nous mangeons du riz nature ce soir.

Is English tonight always ce soir?

Use ce soir for the evening. Cette nuit refers to nighttime (after bedtime). For late tonight, say ce soir tard. Avoid aujourd’hui soir in standard French; just use ce soir.

If someone asks in French Qu’est-ce que vous mangez ce soir ?, can I just answer Du riz nature?

Yes. Short, elliptical answers with only the essential noun phrase are natural in French:

— Qu’est-ce que vous mangez ce soir ?
— Du riz nature.