Маленькая снежинка села мне на лоб и сразу исчезла.

Because it describes снежинка, and снежинка is:

• feminine
• singular
• nominative case (the subject of the sentence)

So the adjective has to agree with it:

• маленький — masculine
• маленькая — feminine
• маленькое — neuter
• маленькие — plural

Here, маленькая снежинка means a small snowflake.

Russian nouns have grammatical gender, and nouns ending in -а / -я are very often feminine. Since снежинка ends in -а, it is feminine.

That matters because other words have to match it, for example:

• маленькая снежинка
• снежинка села
• снежинка исчезла

Both past-tense verbs are in the feminine singular form because the subject is feminine.

Literally, yes: села is the feminine past tense of сесть, which often means to sit down.

But in this sentence, it is used in a natural extended sense: to land / to settle onto something.

So снежинка села мне на лоб means something like:

• The snowflake landed on my forehead
• The snowflake settled on my forehead

Russian often uses сесть this way for birds, insects, and even things like flakes that come to rest somewhere.

This is a very common Russian pattern with body parts.

Instead of saying on my forehead, Russian often says something more like onto me, on the forehead:

• мне на лоб — literally to me onto the forehead

The pronoun мне is dative case of я and marks the person affected. This is very natural in Russian with body parts:

• Он посмотрел мне в глаза — He looked me in the eyes
• Она взяла меня за руку — She took me by the hand
• Снег упал мне на плечо — Snow fell on my shoulder

You could say на мой лоб, but мне на лоб sounds more idiomatic here.

Because Russian changes case depending on whether you mean movement to a place or location at a place.

With на:

• на + accusative = movement onto
• на + prepositional = location on

So:

• села на лоб — landed onto the forehead → movement, so лоб is accusative
• лежит на лбу — is lying on the forehead → location, so лбу is prepositional

In your sentence, the snowflake is moving onto the forehead, so на лоб is correct.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject is снежинка, which is feminine singular, so the verbs are:

• села — feminine singular past
• исчезла — feminine singular past

Compare:

• Мальчик сел и исчез. — masculine
• Девочка села и исчезла. — feminine
• Окно исчезло. — neuter
• Люди сели и исчезли. — plural

Both can often be translated as disappeared / vanished, but they are not always identical.

• исчезла focuses on ceasing to be visible / vanishing
• пропала can mean went missing / was lost / disappeared

In this sentence, исчезла is a very natural choice because the snowflake melted or vanished from sight almost immediately.

So сразу исчезла gives the feeling of it instantly vanished.

Сразу means immediately / right away / at once.

Here it modifies исчезла:

• и сразу исчезла — and immediately disappeared

Its position is fairly flexible. For example:

• Маленькая снежинка села мне на лоб и сразу исчезла.
• Маленькая снежинка сразу села мне на лоб и исчезла.
• Сразу маленькая снежинка села мне на лоб... — possible, but much less natural in this context

The original word order sounds natural because сразу clearly goes with the second action.

Yes. Both verbs present the actions as completed single events.

• села comes from сесть, a perfective verb
• исчезла is understood as a completed act of disappearing, also giving a perfective sense

That fits the sentence well: a snowflake landed, then vanished.

Russian often uses perfective verbs in storytelling when describing one completed event after another.

Russian has no articles, so there is no direct equivalent of a or the.

So маленькая снежинка can mean:

• a small snowflake
• the small snowflake

Which one is meant depends on context. In this sentence, English naturally uses a small snowflake.

Yes, this is a very normal narrative word order.

• Маленькая снежинка — the subject
• села мне на лоб — the first action
• и сразу исчезла — the second action

Russian word order is more flexible than English, so you can rearrange parts for emphasis. For example:

• Мне на лоб села маленькая снежинка и сразу исчезла.
• Сразу исчезла маленькая снежинка, которая села мне на лоб.

But the original sentence is the most straightforward and natural way to tell the event.

Yes, и means and, but in a sentence like this it also naturally conveys sequence:

• she landed on my forehead and immediately disappeared

Russian does not need a separate word for then here, because the order of the verbs already shows what happened first and second.

So и links the two completed actions in a simple narrative way.

The main stresses are:

• Ма́ленькая
• снежи́нка
• се́ла
• мне
• на лоб
• и сра́зу
• исче́зла

A helpful pronunciation guide:

MA-len-kaya snye-ZHEEN-ka SYE-la mnye na lop i SRA-zu is-CHEZ-la

A few notes:

• мне is pronounced roughly like mnye
• снежинка has stress on -жи́н-
• села has stress on the first syllable: се́ла
• final voiced consonants often devoice in pronunciation, so лоб sounds closer to lop

It feels somewhat small and delicate, but in modern Russian снежинка is the normal everyday word for snowflake.

It is related to снег (snow), but learners should treat снежинка simply as the standard vocabulary word, not as a special cute form you need to avoid.

So:

• снег — snow
• снежинка — snowflake

In this sentence, маленькая снежинка is perfectly natural, even though a snowflake is already something small. The adjective adds a gentle, vivid image.