Утром на крыше висели длинные сосульки, а под окном вырос высокий сугроб.

Russian often uses the instrumental case for parts of the day when they function like adverbs of time. So утром means in the morning. This is a very common pattern:

• утром — in the morning
• днём — in the daytime / during the day
• вечером — in the evening
• ночью — at night

Here, the instrumental does not mean an instrument; it is just the standard form in this time expression.

Because this sentence describes location, not movement.

With на:

• на + prepositional = location, on / at
• на + accusative = motion toward a place, onto / to

So:

• на крыше = on the roof where something already is
• на крышу = onto the roof with movement

Since the icicles are hanging there, Russian uses на крыше.

Окном is instrumental singular of окно.

The preposition под works like this:

• под + instrumental = location, under
• под + accusative = movement to a position under something

So:

• под окном = under the window (location)
• под окно = to under the window (movement, much less common in everyday use)

Here there is no movement toward the window; the snowdrift is located there, so Russian uses the instrumental: под окном.

In the past tense, Russian verbs agree with the subject in number, and in the singular they also show gender.

• сосульки is plural, so the verb is plural: висели
• сугроб is masculine singular, so the verb is masculine singular: вырос

So the verb endings tell you what the subject is like:

• plural subject → plural past tense
• masculine singular subject → masculine singular past tense

Because adjectives must agree with the nouns they describe in case, number, and gender.

• длинные сосульки: сосульки is nominative plural, so длинные is nominative plural
• высокий сугроб: сугроб is nominative masculine singular, so высокий is nominative masculine singular

This agreement is one of the core features of Russian grammar.

Yes, Russian often uses вырасти literally with things that increase or build up, including natural formations like a snowdrift.

Here вырос is the past tense of the perfective verb вырасти. It suggests a completed result: by morning, a tall snowdrift had formed / built up under the window.

So in English, depending on context, you might translate it as:

• grew
• had formed
• had built up
• appeared

The Russian verb sounds natural here even if English may prefer a different wording.

А often connects two related facts while also setting them side by side. It can feel like and, while, or whereas, depending on context.

In this sentence, а links two observations:

• on the roof there were long icicles
• under the window there was a tall snowdrift

It gives a slight contrast or parallel structure: one thing is on the roof, another is under the window. Using и would also join the ideas, but а sounds more natural for this kind of scene-setting comparison.

Russian word order is much more flexible than English word order. The subject does not always have to come before the verb.

In this sentence, the order helps present the setting first:

• Утром — time
• на крыше — place
• висели длинные сосульки — what was there

And then:

• под окном — place
• вырос высокий сугроб — what appeared there

This is a very natural Russian way to describe a scene: first give the location, then mention what is found there. The subjects сосульки and сугроб are still the subjects even though they come later.

Both are in the nominative case, because they are the subjects of their clauses.

• сосульки — nominative plural
• сугроб — nominative singular

Even though they appear after the verb, they are still the grammatical subjects, and Russian marks that with the nominative.

Russian has no articles. There is no direct equivalent of English a/an or the.

So:

• длинные сосульки can mean long icicles or the long icicles, depending on context
• высокий сугроб can mean a tall snowdrift or the tall snowdrift

You understand definiteness from context, not from an article.