Por descuido, dejé mi celular en la mesa.

Por descuido literally means “because of carelessness / due to an oversight.” It’s a fixed, common way to explain that something happened accidentally or without paying attention.

• Por + noun often expresses cause: por error, por accidente, por miedo, por prisa.
  So Por descuido, dejé mi celular… = “Out of carelessness / By accident, I left my phone…”.

They’re close, but not identical.

• Por accidente = accidental in a more general, neutral way (an unintended event).
• Por descuido = specifically suggests lack of attention/forgetfulness (you weren’t careful).
  Leaving a phone somewhere is very natural with por descuido.

Because Por descuido is an introductory phrase. In Spanish, it’s common to set off an introductory adverbial phrase with a comma:

• Por descuido, …
• De repente, …
• Sin querer, …
  You can sometimes omit the comma in very short sentences, but here it’s standard and clear.

Dejé is preterite (pretérito indefinido), 1st person singular: I left.
Preterite is used because leaving the phone is a completed, specific action in the past (a single event).
If you used imperfect (dejaba), it would suggest a habitual or ongoing past context:

• Siempre dejaba el celular en la mesa. = “I used to leave my phone on the table (habitually).”

The accent in dejé marks the stressed syllable and distinguishes the form:

• dejé = I left (preterite)
• deje (no accent) = can be a present subjunctive form ((that) I/he/she leave) or a formal command (leave!), depending on context.
  In dejé, the stress is on the last syllable: de-JÉ.

Dejar can mean both:

• to leave (something somewhere): Dejé mi celular en la mesa.
• to let / allow: Déjame ver. (“Let me see.”)
  The pattern here is dejar + object + location (mi celular
  • en la mesa), which clearly signals “leave (behind)”.

Both can be possible, but they feel different:

• dejé mi celular = very explicit: my phone (most natural if you’re emphasizing it was yours).
• dejé el celular = can work if it’s already obvious whose phone it is from context (Spanish often uses el/la where English uses a possessive).
  In isolation, mi celular is the safer, clearer choice.

In Latin America, celular is extremely common and natural.

• móvil is more typical in Spain (though many people in Latin America will understand it).
• teléfono is universal, but can sound more general; celular is specifically a mobile phone.
  So this sentence is very Latin America–friendly.

Both can be correct, but they emphasize slightly different spatial ideas:

• en la mesa is commonly used to mean “on the table” in everyday Spanish.
• sobre la mesa is more explicitly “on top of the table / on the surface.”
  For leaving an object on a table, en la mesa is perfectly idiomatic.

Yes, and it’s very common. The difference is focus/responsibility:

• Por descuido, dejé mi celular en la mesa. = “I left it (I did the action), due to carelessness.”
• Se me olvidó mi celular en la mesa. = “I forgot my phone on the table” (frames it as something that happened to you; very natural Spanish).
  Both convey the same basic situation, but se me olvidó often sounds more conversational and less blame-focused.

Yes. All of these are possible, with slightly different emphasis:

• Por descuido, dejé mi celular en la mesa. (sets the reason first)
• Dejé mi celular en la mesa por descuido. (states the fact first, then the reason)
• Dejé mi celular en la mesa, por descuido. (pause adds an afterthought feeling)
  The original order is very natural.

descuido is pronounced roughly des-KWEE-doh (with Spanish vowels). It comes from descuidar (to neglect / to be careless).

• un descuido = an oversight, a lapse in attention
  So por descuido is like “because of an oversight.”