"The boy sees the baby."
Translation:La knabo vidas la bebon.
It (the "n" ending) is the accusative, which marks the object, because Esperanto has flexible word order, so "La knabo vidas la bebon" is the same as "La bebon vidas la knabo", or "Vidas la bebon la knabo", or "Vidas la knabo la bebon", or "La knabo la bebon vidas", or "La bebon la knabo vidas". In Esperanto the accusative is never used with "estas" because "I am the person", and "The person is I" are the same, English does have the accusative for pronouns though ("me" is the accusative form of "I"), and it should not be used with "to be" (example: "That is I", not "That is me"). English always has SVO (Subject-Verb-Object) word order, so "The boy sees the baby" is different from "The baby sees the boy".
A: I think you mean rigardas.
B: Rigardi = to look at/watch; vidi = to see.
When you rigardas something, it is over a period of time, e.g. la bebon mi rigardos atenteme: I will watch the baby intently.
When you vidas something, it happens in a moment, e.g. subite mi vidis: suddenly I saw.
Ke tio ĉi helpis, mi esperas!