Мы чуть было не съели червивую сыроежку.

It means almost or came very close to doing something.

So Мы чуть было не съели червивую сыроежку means that we nearly ate the mushroom, but in the end we did not.

A useful point: even though the sentence contains не, the whole expression чуть было не + past verb usually means something almost happened.

Compare:

• Мы не съели сыроежку = We did not eat the russula
• Мы чуть было не съели сыроежку = We almost ate the russula

By itself, yes, не usually negates a verb. But in the fixed pattern чуть не or чуть было не, it does not simply mean ordinary negation. The whole expression functions idiomatically as almost.

So here:

• съели = ate
• не съели = did not eat
• чуть было не съели = almost ate

This is one of those cases where you should learn the whole chunk together.

Both can mean almost.

• чуть не съели = we almost ate
• чуть было не съели = also we almost ate

But чуть было не often sounds a bit more vivid, emotional, or narrative, as if the speaker is emphasizing how close the event was. It can feel a little more dramatic: we very nearly did.

In everyday speech, both are common.

Because съесть is the perfective verb, and here Russian is talking about a near-completed single action: we almost ended up eating it.

• есть / ели = imperfective, to eat / were eating / ate in a process-oriented sense
• съесть / съели = perfective, to eat up / eat as a completed event

With чуть было не, Russian often uses the perfective when talking about a specific action that almost happened.

So:

• чуть было не ели would sound odd here
• чуть было не съели is the natural choice

Because the subject is мы = we.

In the past tense, Russian verbs agree in gender/number with the subject:

• я съел / съела
• ты съел / съела
• он съел
• она съела
• мы съели
• они съели

So съели is the correct past-tense plural form for мы.

Because it is the direct object of the verb съели, so it is in the accusative case.

The dictionary form is:

• сыроежка = a russula mushroom

Its accusative singular is:

• сыроежку

Since съесть что? = to eat what?, the object must be in the accusative:

• съели сыроежку

Because it has to agree with сыроежку in gender, number, and case.

The noun is:

• сыроежка — feminine singular
• here it is in the accusative singular: сыроежку

So the adjective червивый changes to the matching form:

• червивая сыроежка = nominative
• червивую сыроежку = accusative

This agreement is completely normal in Russian adjective-noun phrases.

Червивый means worm-eaten, wormy, or infested with worms.

So червивая сыроежка is a russula mushroom that has worms in it or has been eaten through by larvae.

In natural English, worm-eaten mushroom may sound more idiomatic than wormy mushroom, but both help you understand the Russian adjective.

Yes, it could be omitted.

Russian often leaves out subject pronouns when they are clear from the verb form. Since съели already tells us the subject is we or they, context usually clarifies it.

So both are possible:

• Мы чуть было не съели червивую сыроежку.
• Чуть было не съели червивую сыроежку.

Including мы can add emphasis, contrast, or clarity.

No, Russian word order is fairly flexible, though some versions sound more natural than others depending on emphasis.

The given sentence is neutral and natural:

• Мы чуть было не съели червивую сыроежку.

You could also hear:

• Мы червивую сыроежку чуть было не съели.
  This puts more focus on the wormy russula
• Червивую сыроежку мы чуть было не съели.
  This strongly highlights the object

The meaning stays basically the same, but the emphasis changes.

It is a feminine noun meaning russula, a type of mushroom.

Its ending -а is a common sign of feminine nouns. That is why:

• nominative singular: сыроежка
• accusative singular: сыроежку

For learners, the important thing is not the botanical detail, but that it behaves like a regular feminine noun in -а.

Normally, no. The standard interpretation is that we nearly ate it, but did not.

That is exactly what чуть было не + past verb expresses: a narrowly avoided event.

So the sentence implies:

• the action came close to happening
• but it was ultimately avoided

In чуть было не, the word было is part of a fixed expression. It is not agreeing with мы.

Historically, it comes from the past tense of быть (to be), but in this construction learners should treat чуть было не as a set phrase meaning almost.

So you do not change it according to the subject:

• Я чуть было не упал = I almost fell
• Она чуть было не забыла = She almost forgot
• Мы чуть было не съели... = We almost ate...

It stays было regardless of who the subject is.