Temporal vs Logical Relationships

Anonymous-
1 Do not lose your hope when a doctor said survival was impossible.
2 Do not lose your hope just because the doctor said survival was impossible.

1 has a temporal relationship. when.
2 has a logical relationship. just because.

Both sentences start with an imperative (a command), which is always in present time. The when-clause is in the past. The two are contradictory.

We don't say "Throw water on the pile of dead leaves when they caught fire (yesterday)". The time the leaves caught fire is long past. It's too late to command anyone to throw water on them.

Because of the problem of time relationships, you have to say Don't lose hope (even) if a doctor says survival is impossible. You can't use when because you don't even know if any doctor is going to say survival is impossible. It's only a future possibility.
___________

As for sentence 2, an imperative is compatible with a because-clause because the word because creates no crucial time relationship with the rest of the sentence.

CJ