Answer/Explanation:
Il y sont allés
"à Dakar en voyage d’affaire" is a place, so "y" is used.
"sont allés" is not split because it is as past tense.
Il est important de leur obéir
The replaced noun includes "aux" followed by not a place. We use "leur" because it is plural and is not a place.
The pronoun precedes the "obéir" verb and not "est" because we are talking about obeying(obéir) the laws, not being(etre) the law.
J’ai lui téléphoné
"à mon cousin" is a person so we use "lui". There is "à" so we need an indirect pronoun.
a. Je veux y répondre.
"veux répondre" can be split by the pronoun because is it the second verb in present tense, not past tense.
b. il y est allé
"est allé" is together because it is the past tense.
Summary:
Pronouns go before a verb.
Past tense verbs are not split. Double verbs in present tense are split.
