The other half of the rule. Drop the -er or -ir and add -ido. There is no separate "-edo": remember that the only place these two groups ever differ is the present "we" form.
-ar → -ado · -er / -ir → -ido
Now bienvenido makes sense: well-come.
He terminado · Lo he terminado · Los he terminado · He perdido · Yo he perdido · Hemos perdido
Terminar is to terminate, but the everyday English word for it is to finish.
The student simplifies the rule to "at the beginning of the sentence" and the teacher pushes back: that simplification will let you down.
Why it matters here: he terminado is a rare case of two changed verbs together. Usually a changed verb is followed by a complete one (quiero terminar, puedo terminar, debo terminar). With two changed verbs there is only one place the little word can go, and "he lo terminado" puts it after a changed verb, which is wrong.
The core of this track. There are two separate groups, and the job is to stop letting a vague sense of gender pick one for you.
And the other set, the little words:
Build the whole little set from lo: change the o to a for feminine, add s for plural.
Lo veo · La veo · Los veo · Las veo · Yo veo · Tú ves · Usted ve · Él ve · Ella ve · Nosotros vemos · Ellos ven · Ellos los ven · Ellos me ven
The difference between tú and te is the same as between he and him, or she and her. English changes those without a thought: he sees, I see him.
The trap is that English you does not change: you see, I see you. Since English gives you no signal, Spanish can confuse you here.
Usted is formal you, a contraction of vuestra merced, "your honour".
Word order in Spanish is much more flexible than in English. The subject words move freely, but the little words do not.
Usted los ve · Los ve usted
Two places where English pulls you the wrong way, plus one dialect note.