Если не чай, то хотя бы воду возьми с собой.

It is a very common Russian pattern meaning If not X, then at least Y.

So:

• Если не чай = If not tea
• то хотя бы воду возьми с собой = then at least take water with you

It often sounds like a compromise or minimum acceptable option: tea would be better, but water is the fallback.

Because если introduces a subordinate clause, and Russian normally separates it from the main clause with a comma:

• Если не чай, то хотя бы воду возьми с собой.

This is true even though the first part is very short.

Хотя бы means at least, even just, or if nothing else.

In this sentence, it softens the request and sets a minimum:

• чай would be preferable
• but вода is the minimum acceptable thing to take

So хотя бы воду = at least water / if nothing else, water.

Because воду is the accusative singular form of вода.

The verb взять / возьми takes a direct object:

• вода = nominative
• воду = accusative

So:

• возьми воду = take water

This is exactly the same kind of object marking as in many other Russian sentences.

Good question: for inanimate masculine singular nouns like чай, the nominative and accusative look the same.

So:

• nominative: чай
• accusative: чай

In this sentence, the first part is also shortened/elliptical, so чай is basically just naming the option tea. Even if you expanded the sentence, the form would still likely be чай, so the case is not visible from the ending.

Возьми is the imperative of the perfective verb взять. It means something like take it / make sure to take it as a single completed action.

That fits this sentence well, because the speaker is telling someone to take water along.

Compare:

• возьми = take it, do it
• бери = take it / go ahead and take it / keep taking it, with a more process-like or habitual feel

In practical terms, возьми is the more natural choice here.

С собой means with you / along with you, literally with oneself.

Russian uses с собой with verbs like take, bring, carry when the person taking something and the person accompanying it are the same person:

• возьми с собой = take with you

Why not с тобой?

• с тобой usually means with you as a separate person
• с собой means with yourself

So in this sentence, с собой is the idiomatic and correct choice.

Not always strictly necessary, but it is very natural.

Russian often uses the pair:

• если ... то ... = if ... then ...

So here:

• Если не чай, то хотя бы воду...

The то makes the contrast clearer and more structured. You may hear sentences without it in casual speech, but with то sounds very standard.

Because Russian often omits words that are obvious from context. This is called ellipsis.

Here, the full idea might be something like:

• Если не возьмёшь чай, то хотя бы воду возьми с собой
• Если не чай будешь брать, то хотя бы воду возьми с собой

But Russian very naturally shortens this to:

• Если не чай, то хотя бы воду возьми с собой

English does this less freely, so it can feel abrupt to learners, but in Russian it is normal.

The word order is flexible. Russian word order often changes for emphasis.

This version:

• Если не чай, то хотя бы воду возьми с собой

puts a little emphasis on воду as the minimum acceptable option.

Other possible arrangements include:

• Если не чай, то возьми с собой хотя бы воду.
• Если не чай, то хотя бы возьми с собой воду.

They all mean roughly the same thing, but the emphasis shifts slightly. The original sentence sounds natural and conversational.

It sounds like a mild command or practical advice, depending on context.

Because of возьми, it is an imperative, so grammatically it is a command. But хотя бы softens it:

• not a harsh order
• more like Well, if not tea, then at least take some water with you

So the tone is usually helpful, persuasive, or slightly insistent rather than strict.