Если у нас останется ещё одно киви, завтра я добавлю его в смузи.

Russian usually expresses possession with у + genitive rather than with a verb like English have.

So у нас останется literally means something like there will remain by us or more naturally we will have left.

A useful way to see the structure is:

• у нас = with us / by us
• останется ещё одно киви = one more kiwi will remain

This possession pattern is extremely common in Russian:

• У меня есть книга = I have a book
• У нас осталось два яблока = We have two apples left

In your sentence, the thing that remains is одно киви, so the verb agrees with that.

This is a very common difference between English and Russian.

In English, after if, we normally use a present form for future meaning:

• If we have one kiwi left, ...

In Russian, future meaning can still appear after если, especially with a perfective verb. That is exactly what happens here:

• останется = future of остаться

So Если у нас останется... is completely normal Russian and means If we end up having left... / If there is ... left.

Also, останется looks like a present-tense form, but because остаться is perfective, this form has future meaning. Perfective verbs in Russian do not have a true present tense.

Because the sentence is about a single completed result: whether, after everything else happens, one kiwi is left over.

• остаться = perfective, to remain / be left as a result
• оставаться = imperfective, to be remaining / keep remaining

Here the speaker is not describing an ongoing state. They are talking about the outcome of a situation: will one kiwi be left or not? That is why останется fits best.

Compare:

• Если у нас останется ещё одно киви... = if one kiwi ends up being left
• Если у нас будет ещё одно киви... = if we happen to have another kiwi

The version with останется specifically suggests left over.

Here ещё means one more / another / an extra, depending on context.

So ещё одно киви means something like:

• one more kiwi
• another kiwi
• one kiwi left over in this particular sentence context

The exact English wording depends on the broader situation, but the core idea is that there is an additional single kiwi.

одно is the neuter form of one.

The noun киви is indeclinable, so its ending does not show gender or case. In this sentence it is being treated as neuter, so the agreeing numeral is одно.

That is why you get:

• одно киви

rather than a changed noun ending.

The important point for a learner is that with indeclinable nouns, the agreement often shows up on surrounding words, not on the noun itself.

Оно is the nominative form, used when the noun is the subject.

But here the kiwi is the direct object of добавлю:

• я добавлю его = I will add it

For a neuter singular pronoun, the accusative form is его. So:

• nominative: оно
• accusative: его

That is why его is correct here.

Also, его is used for both masculine and neuter singular in this form, so seeing его does not automatically mean the noun is masculine.

Because киви is an indeclinable noun in Russian.

That means it keeps the same form in different cases:

• одно киви
• нет киви
• добавлю киви
• о киви

The grammatical information is shown by other words in the sentence, such as:

• одно for agreement
• the verb
• pronouns like его
• prepositions

This is common with many borrowed words in Russian.

Because добавлю is perfective and refers to a single completed action in the future: I will add it.

• добавить = perfective, to add once, successfully/completely
• добавлять = imperfective, to be adding / add repeatedly / add in general

In this sentence, the speaker means one specific future action, so добавлю is the natural choice.

Compare:

• Завтра я добавлю его в смузи = tomorrow I’ll add it to the smoothie
• Завтра я буду добавлять его в смузи = tomorrow I’ll be adding it to the smoothie

The second version sounds more ongoing or process-focused, which is less natural here.

The verb добавить often takes the pattern добавить что-то в что-то when you mean add something into something.

So:

• добавлю его в смузи = I’ll add it into the smoothie

The preposition в here expresses movement into something.

As for смузи, it is also an indeclinable borrowed noun, so its form stays the same. Even though the case is determined by the preposition and the verb, the word itself does not visibly change.

Yes, Russian word order is flexible.

Here завтра belongs naturally to the main clause:

• Если у нас останется ещё одно киви, завтра я добавлю его в смузи.

This order makes the structure clear:

1. condition first
2. result second

But other placements are possible, for example:

• Завтра, если у нас останется ещё одно киви, я добавлю его в смузи.
• Если у нас останется ещё одно киви, я завтра добавлю его в смузи.

These versions are all grammatical, but the emphasis shifts slightly.

Because Если у нас останется ещё одно киви is a subordinate clause introduced by если.

In Russian, subordinate clauses are separated from the main clause with a comma:

• Если ..., ...

So the comma marks the boundary between:

• the condition: Если у нас останется ещё одно киви
• the main statement: завтра я добавлю его в смузи

This is standard Russian punctuation.