Nieva en diciembre.

Because the verb is nevar and it has a stem change in the present tense: e → ie in stressed forms.

So:

• nieva
• nievan

But:

• nevamos
• neváis in Spain

This is a common pattern in Spanish, like:

• pensar → piensa
• cerrar → cierra

So nieva is the correct present-tense form.

Spanish often leaves out the subject pronoun, especially when the verb form already makes it clear.

With weather verbs like nevar, English uses it as a dummy subject: It snows.
Spanish usually does not use a subject here. You simply say:

• Nieva
• Llueve
• Hace frío

So Nieva en diciembre is natural, and adding a subject would usually sound unnecessary.

It is a weather verb, meaning a verb commonly used to talk about weather conditions.

Other common weather expressions include:

• llover = to rain
• granizar = to hail
• hacer calor = to be hot
• hacer frío = to be cold

Weather verbs are often used in the third person singular, because they describe a general condition rather than something a person does:

• Llueve
• Nieva

Spanish uses en to say in with months.

So:

• en diciembre = in December
• en enero = in January
• en verano = in summer

This is different from English in some time expressions, but for months, en is the normal choice.

No. In Spanish, months are not normally capitalized unless they begin a sentence or are part of a title.

So:

• Nieva en diciembre. ✅
• Nieva en Diciembre. ❌

The same rule applies to days of the week and seasons:

• lunes
• abril
• invierno

By default, Nieva en diciembre usually sounds like a general or habitual statement: It snows in December.

Spanish present tense often expresses things that happen regularly or typically.

If you want to say it is happening right now, Spanish more often uses:

• Está nevando en diciembre = It is snowing in December

That said, context matters. Nieva can sometimes describe what is happening now, but without context it usually feels more general.

Yes, but it means something a little different.

• Nieva en diciembre = It snows in December
• Hay nieve en diciembre = There is snow in December

The first talks about snow falling.
The second talks about snow being present on the ground or in the area.

So they are related, but not interchangeable in every situation.

Because weather verbs like nevar are normally used in the third person singular when describing weather.

So you say:

• Nieva
• Llovió
• Graniza

This is similar to English it snows, where it does not refer to a real thing. In Spanish, that kind of subject is usually omitted, but the singular verb remains.

Normally, no, when talking about weather.

You usually do not say that a person nieva. The standard weather use is impersonal:

• Nieva mucho aquí.

However, in poetic or figurative language, Spanish can sometimes use weather verbs creatively, but that is not the normal everyday pattern learners should focus on.

A Latin American pronunciation would be approximately:

NYEH-bah en dee-see-EM-breh

A few helpful points:

• nie- sounds like nyeh
• v and b sound very similar in Spanish
• ci in most of Latin America sounds like see
• diciembre has the stress on -em-: di-ciem-bre

So the stress pattern is:

• NIE-va
• di-ci-EM-bre

Normally, no. You usually say simply:

• en diciembre

Months in Spanish normally do not take the article after en in expressions like this.

Correct:

• Nieva en diciembre.
• Viajamos en mayo.

Using el here would sound unnatural in standard Spanish.

You can use the same pattern with other months or weather verbs:

• Nieva en enero.
• Llueve en abril.
• Hace calor en agosto.
• Hace frío en diciembre.

This pattern is useful:

[weather expression] + en + [month]

It is a simple and common way to talk about typical weather during different times of the year.