Nel caso tu dimentichi la chiave, ho una copia sulla scrivania.

Italian uses the congiuntivo (subjunctive) in clauses introduced by nel caso (che) to express a hypothetical or uncertain event. Even if you’re talking about a possible future action, you still choose the present subjunctive (dimentichi) to signal that it’s not a certainty.

• The pattern is nel caso (che) + subjunctive.
• Note: the 2nd-person singular present indicative and present subjunctive share the same form (tu dimentichi); you identify the subjunctive by the trigger (nel caso).
• Using the future indicative (dimenticherai) would lose the nuance of “just in case.”

Both nel caso and in caso translate as in case, but:

• You can say nel caso che tu dimentichi or in caso che tu dimentichi with the subjunctive.
• che is optional when you express the subject (tu); you may drop che and simply say nel caso tu dimentichi or even nel caso dimentichi.
• If you omit both che and tu, you get in caso dimentichi, which is still understood but less common without context.

No. Italian verb endings already indicate the subject, so pronouns are often omitted. Here:

• Including tu adds clarity or emphasis.
• You can equally say nel caso dimentichi la chiave.
• Dropping tu and che gives nel caso dimentichi, which remains correct.

sulla is a fusion of su (on) + la (the). In Italian you usually combine prepositions with definite articles:

• su + la = sulla
• su + il = sul, su + i = sui, etc.
  The definite article is required when indicating location on furniture or a specific surface—sulla scrivania means on the desk.

Yes, se dimentichi la chiave (indicative) means if you forget the key, stating a straightforward condition.

• se + indicative presents a real or likely condition.
• nel caso + subjunctive adds the sense of precaution—just in case, treating the forgetfulness as a hypothetical scenario.

Absolutely. casomai is a colloquial alternative meaning in case. It also takes the subjunctive:

• casomai dimentichi la chiave, ho una copia sulla scrivania.

Because the speaker already has a copy on the desk right now.

• ho una copia expresses present possession.
• avrò una copia would imply “I will have a copy (later),” which changes the meaning.