Мы будем собирать грибы, разве что начнётся дождь.

Because собирать is an imperfective verb. In Russian, the future of an imperfective verb is formed with быть in the future + infinitive:

• будем собирать = we will be picking / we will pick

This form often suggests a process, activity, or ongoing action rather than a single completed result.

Compare:

• Мы будем собирать грибы. = We’ll be mushroom-picking / we’ll pick mushrooms.
• Мы соберём грибы. = We’ll gather the mushrooms / we’ll pick them all and finish.

So будем собирать focuses on the activity itself.

These are an aspect pair:

• собирать = imperfective
• собрать = perfective

In this sentence, собирать is used because the speaker is talking about the plan or activity of mushroom-picking, not the completed result.

• будем собирать грибы = we’ll be picking mushrooms
• соберём грибы = we’ll pick/gather the mushrooms completely

A native English speaker can think of it like this:

• imperfective = emphasis on the process or event
• perfective = emphasis on completion or result

Here, going out to pick mushrooms is the main idea, so imperfective sounds natural.

Грибы is the accusative plural, but for inanimate masculine plural nouns, the accusative looks the same as the nominative.

• nominative plural: грибы
• accusative plural: грибы

It is accusative here because собирать takes a direct object: you collect or pick something.

Compare:

• Мы собираем грибы. = We pick mushrooms.
• Мы видим грибы. = We see mushrooms.

If the noun were animate, the accusative plural would usually look different.

Russian has no articles like the, a, or some.

So грибы can mean different things depending on context:

• mushrooms
• some mushrooms
• the mushrooms

In this sentence, the natural meaning is just mushrooms in a general sense: they are going mushroom-picking.

Russian leaves this kind of distinction to context, word order, and the situation.

Разве что means something like:

• unless
• except maybe if
• the only exception would be if

So:

• Мы будем собирать грибы, разве что начнётся дождь. = We’ll go picking mushrooms, unless it starts raining.

It introduces an exception to what was just said.

This expression is common in conversational Russian and often sounds slightly more nuanced than a simple если не structure. It suggests: our plan stands, except in this one case.

Yes, in many situations you could say:

• Мы будем собирать грибы, если не начнётся дождь.

This also means We’ll pick mushrooms if it doesn’t start raining or unless it starts raining.

But there is a slight nuance:

• если не = more straightforward condition
• разве что = more like except perhaps if, highlighting the exception

So разве что can sound a bit more idiomatic and conversational here.

Начнётся is the perfective future of начаться and means will start / will begin.

That fits well because the sentence refers to a single event that would interrupt the plan:

• разве что начнётся дождь = unless it starts raining

The form будет начинаться is the future of the imperfective verb начинаться, and it would usually describe a process or repeated/ongoing beginning, which does not sound natural here.

So Russian prefers the perfective verb because the important idea is the moment when the rain begins.

Because дождь is the subject of начнётся.

Literally, this part means:

• дождь начнётся = the rain will start

Since дождь is the thing doing the action of beginning, it stays in the nominative.

Because разве что начнётся дождь is a separate clause-like element expressing an exception.

Russian punctuation often uses a comma before conjunctions and connective expressions that introduce another clause or a clause-like addition. Here the comma helps separate:

• main statement: Мы будем собирать грибы
• exception: разве что начнётся дождь

So the comma is standard and natural.

Yes, it is natural, though it is fairly literal: We will be picking mushrooms.

Depending on context, Russian might also use other expressions, for example:

• Мы пойдём за грибами. = We’ll go mushroom-picking.
  (literally: We’ll go for mushrooms.)

But Мы будем собирать грибы is absolutely correct and clearly emphasizes the actual activity of collecting them.

Yes, Russian word order is flexible, though the neutral version here is very natural:

• Мы будем собирать грибы, разве что начнётся дождь.

You might also hear:

• Разве что дождь начнётся, мы будем собирать грибы.
• Мы, разве что начнётся дождь, будем собирать грибы.

But these are less neutral and may sound more marked, literary, or context-dependent.

For a learner, the original order is the best one to remember because it is clear and idiomatic:

1. statement of the plan
2. exception to the plan