Breakdown of Fue hoy cuando la vi desenchufar la máquina.
ser
to be
ver
to see / to look / appear
hoy
today
la
the (feminine)
Questions & Answers about Fue hoy cuando la vi desenchufar la máquina.
Could I say Fue hoy que la vi... instead of using cuando?
Yes. In everyday Latin American Spanish, using que as the relative word in cleft sentences is very common, even when referring to time. So, Fue hoy que la vi... is perfectly natural and widely accepted, though cuando is the traditional choice and sounds slightly more precise for time.
Why is the verb ser conjugated in the preterite (fue) here? Could it be in the present tense?
In cleft sentences, the verb ser usually matches the tense of the main action. Since you saw her in the past (vi is preterite), ser is also in the preterite (fue). While using the present tense (Es hoy cuando la vi...) is sometimes heard, matching the past tense is much more common and sounds more natural.
Why is it la vi and not le vi? Isn't she the person doing the unplugging?
With verbs of perception like ver (to see) and oír (to hear), the person you see or hear is treated as the direct object. Therefore, we use the direct object pronoun la (her). You would use the indirect object pronoun le only if you were doing a transfer to or for her, rather than just seeing her.
Why is desenchufar in the infinitive? In English, I might say I saw her unplugging.