Грибы съедобные, разве что этот мухомор.

In Russian, the verb to be is normally omitted in the present tense.

So:

• Грибы съедобные = The mushrooms are edible
• literally: Mushrooms edible

This is completely normal Russian.
You would only see a form of быть in the past or future:

• Грибы были съедобные = The mushrooms were edible
• Грибы будут съедобные = The mushrooms will be edible

Russian has no articles like a or the, so context tells you.

Грибы can mean:

• mushrooms in general
• the mushrooms in a particular situation
• some mushrooms depending on context

In this sentence, it most likely means a specific set of mushrooms that the speaker is talking about, for example mushrooms someone has found or collected.

Because it agrees with грибы, which is plural.

• гриб = mushroom
• грибы = mushrooms

So the adjective must also be plural:

• съедобный = edible (masculine singular)
• съедобная = edible (feminine singular)
• съедобное = edible (neuter singular)
• съедобные = edible (plural)

Both are possible, but they sound a little different.

• Грибы съедобные sounds natural and conversational.
• Грибы съедобны is also correct, but it can sound more bookish, formal, or categorical.

With adjectives like съедобный, the full form is very common when you are describing something as belonging to a class or type.

So:

• Грибы съедобные = very natural in everyday speech
• Грибы съедобны = also correct, but a bit more formal

Here разве что means something like:

• except maybe
• unless perhaps
• with the possible exception of

So the sentence means that the mushrooms are edible, but the speaker is singling out one possible exception:

• Грибы съедобные, разве что этот мухомор.
• The mushrooms are edible, except maybe this fly agaric.

A key nuance is that разве что often sounds softer or less absolute than a plain except.

Because разве что introduces an added exception to the first statement.

The first part is already a complete idea:

• Грибы съедобные = The mushrooms are edible

Then the speaker adds a qualifying exception:

• разве что этот мухомор = except maybe this fly agaric

So the comma marks that extra afterthought or exception.

Because разве что is not a preposition like кроме that automatically requires the genitive.

Compare:

• кроме этого мухомора = except for this fly agaric
  Here кроме requires genitive, so you get этого мухомора.

But with разве что, the structure is more elliptical and looser. It is like the speaker is adding a separate exception idea:

• разве что этот мухомор [не съедобный / не съедобен]

Also, with a masculine inanimate noun like мухомор, the nominative and accusative forms look the same anyway:

• этот мухомор = nominative form
• этот мухомор = accusative form as well, because it is inanimate

So the important point is: разве что does not force the genitive the way кроме does.

Этот means this and points to a specific mushroom.

It tells you the speaker means one particular mushroom, not мухоморы in general.

So:

• разве что этот мухомор = except maybe this fly agaric
• разве что мухомор would sound more generic or less clearly tied to one specific mushroom

In real life, this often suggests the speaker is pointing at it or has already identified it.

Мухомор usually refers to the well-known poisonous mushroom called fly agaric, especially the classic red mushroom with white spots.

In everyday Russian, it strongly suggests a poisonous or dangerous mushroom. So it is a very natural choice for an obvious exception in a sentence about what is edible.

Learners sometimes notice that the word looks unusual. Historically it is connected with муха (fly) and морить (to kill off / poison), so the idea is something like fly-killer.

Yes, absolutely.

• Грибы съедобные, кроме этого мухомора = The mushrooms are edible, except for this fly agaric

This version is more direct and matter-of-fact.

By contrast:

• Грибы съедобные, разве что этот мухомор

sounds more conversational and a bit softer, as if the speaker is saying:

• Well, they’re edible — except maybe this one
• The only possible exception is this one

So both are correct, but the tone is different.

The ъ is the hard sign. It separates the prefix-like с- from the following е and makes the е sound more like ye.

So съедобные is pronounced roughly like:

• sye-DOB-ny-ye

The hard sign prevents the с from simply merging smoothly into a softened sound. In practical terms, it helps you hear the beginning as с + е / ye rather than treating it as one softer unit.

For learners, the main thing to remember is:

• съе- sounds like sye-

There is a slight sense of caution or tentativeness because of разве что.

The speaker is basically saying:

• These mushrooms are edible — well, except maybe this fly agaric

So the main statement is confident, but the exception is presented a little less bluntly than it would be with a stronger, more direct wording.

If the speaker wanted to sound more flatly certain, they might use something more direct like:

• Грибы съедобные, кроме этого мухомора.

That sounds more like a firm factual exception.