Écrire, parler and donner are all verbs that demand this question at some point. Without getting too bogged down in the grammatical details, here are the ways to differentiate between the two sentence structures.

The sentence structure that results in le/la/l’ instead of lui does not have a preposition before the object. What is a preposition? It is a class of words that primarily denote the position of an object in relation to another. E.g. The ball is under the table; Children sometimes urinate in swimming pools; Jesus came to the world as a man; He threw the cat over the fence.

Look at the following examples in English:

He gave the jacket to her –> ‘her’ is an indirect object as the preposition ‘to’ appears before it.

He gave her the jacket –> ‘her’ is a direct object as no preposition appears before it, so it is not separated from the verb ‘gave’

Of course, this presupposes knowing the grammatical structure of the phrase you are trying to translate. Let’s use ‘to give’ as an example:

to give something = donner quelque chose (direct – no preposition) || to give to someone = donner à qulequ’un (indirect) || to give something to someone = donner quelque chose à qulequ’un

Je donne le veste à Sophie –> Je le donne à Sophie –> Je le donne [à] lui –> Je le lui donne

J’écris un email à Sophie –> Je l‘écris à Sophie –> Je l’écris [à] lui –> Je le lui écris

‘À lui’ is redundant and not written because the ‘lui’ already implies the indirect nature of the object. However, ‘à lui’ is correct when it acts as a disjunctive pronoun (more on that in the future).

The structure of parler is parler à quelqu’un – Which type of pronoun is needed in this case? Rewrite this sentence by replacing the bit in bold with the correct pronoun: Je parle souvent à Letishia

More on direct and indirect object pronouns can be found here and here.