Если ночью будет холодно, рассаду лучше оставить на подоконнике.

Ночью is the instrumental singular of ночь, and in Russian this form is very commonly used as a time expression meaning at night or during the night.

This is a standard pattern with parts of the day:

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

So Если ночью будет холодно... means If it is cold at night / during the night...

Because холодно is not an adjective here. It is a predicative word meaning cold in the sense of it is cold.

Compare:

• холодный = cold describing a noun
  
  • холодный воздух = cold air
• холодно = it is cold / cold conditions

So:

• Ночью будет холодно = It will be cold at night
• Ночь будет холодной would mean something more like The night will be cold, where ночь is the subject

Both can be correct in different contexts, but this sentence uses the normal impersonal weather-style expression.

This is an important difference between Russian and English.

In Russian, when the condition is about the future, you normally use the future tense after если:

• Если ночью будет холодно, ...

In English, future meaning after if is usually expressed with the present:

• If it is cold tonight, ...

So Russian and English work differently here. Russian does not avoid the future tense after если the way English avoids will after if.

There is no real subject here. This is an impersonal construction.

Russian often talks about weather, temperature, light, comfort, and general conditions without a subject:

• Холодно = It is cold
• Темно = It is dark
• Жарко = It is hot
• Трудно = It is difficult

In the future, this becomes:

• Будет холодно = It will be cold

So будет is in the default 3rd person singular form, even though there is no noun subject.

Рассаду is the accusative singular form of рассада.

• dictionary form: рассада
• accusative singular: рассаду

It has -у because рассада is a feminine noun ending in -а, and those usually change -а to -у in the accusative singular.

It is accusative here because it is the direct object of оставить:

• оставить что? → рассаду

Usually рассада is a collective or mass-style noun. It often refers to seedlings as a group, not just one single plant.

So in English it may be translated as:

• seedlings
• young plants
• seedling trays / planted seedlings, depending on context

Even though the Russian form is singular, the meaning can easily be collective.

Yes, this is an impersonal recommendation structure.

Лучше + infinitive means:

• it is better to...
• you had better...
• it would be better to...

So:

• рассаду лучше оставить на подоконнике = it is better to leave the seedlings on the windowsill

Russian often leaves the person unstated when it is obvious from context. If you wanted, you could make the person explicit with the dative:

• Вам лучше оставить рассаду на подоконнике = You’d better leave the seedlings on the windowsill

Оставить is the perfective infinitive. It presents the action as a single complete decision or result: leave it there.

That fits the meaning here: the advice is about one concrete action in response to a condition.

• лучше оставить = better to leave it
• лучше оставлять would sound more like a repeated or habitual action: better to keep leaving it / better to leave it regularly

So the perfective form is the natural choice in this sentence.

Because на подоконнике expresses location, not movement toward a destination.

After на:

• на + accusative usually means motion onto something
  
  • поставить на подоконник = put onto the windowsill
• на + prepositional usually means location on something
  
  • на подоконнике = on the windowsill

Here the idea is leave the seedlings on the windowsill, meaning let them remain there. That is why Russian uses the location form:

• на подоконнике

It is the prepositional singular of подоконник.

• dictionary form: подоконник
• prepositional singular: подоконнике

It appears in the prepositional because the preposition на is being used in the meaning of on / at a location:

• на столе = on the table
• на балконе = on the balcony
• на подоконнике = on the windowsill

Yes. In Russian, a clause introduced by если is a subordinate clause, and it is separated from the main clause by a comma.

So this is correct:

• Если ночью будет холодно, рассаду лучше оставить на подоконнике.

This would also still need a comma if the order changed:

• Рассаду лучше оставить на подоконнике, если ночью будет холодно.

Yes. Russian word order is flexible, although different orders can change the emphasis slightly.

The original sentence starts with the condition, which is very natural:

• Если ночью будет холодно, рассаду лучше оставить на подоконнике.

You could also say:

• Рассаду лучше оставить на подоконнике, если ночью будет холодно.

The basic meaning stays the same. The original version simply foregrounds the condition first: if it gets cold at night...

Yes, that is possible. Both are grammatical.

• Если ночью будет холодно sounds very natural and neutral.
• Если будет холодно ночью is also understandable, but the placement of ночью gives it a slightly different rhythm or emphasis.

Russian word order often shifts to highlight different parts of the sentence, but in this context the original version is probably the most standard and natural.