Whether you’re dealing with one object pronoun or more, reflexives, direct or indirect objects, there are only four scenarios you need to keep in mind in order to determine where to place it or them. When dealing with double object pronouns, they must move together, in tandem, indirect first, direct second. In the se construction known formally as the dative of interest (dumbed down in every textbook I’ve seen for twenty years to euphemisms such as “how to talk about unexpected or unfortunate events,” se comes first, then the indirect object, which indicates the person(s) impacted by the action.
Without any more preamble, here are the possible positions for object pronouns. All the examples will use double object pronouns, but even if there were only one, these are all the possible positions for object pronouns.
When there is only one conjugated verb, there are no options. The object pronoun(s) must be placed directly before the verb. If the sentence is negative, object pronouns are still placed directly before the verb (after the word no):
Él te lo da.
Él no te lo da.
When an auxiliary verb introduces an infinitive, the object pronoun(s) may be placed either as in the First Scenario (before the conjugated verb, in this case, the auxiliary) or attached to the end of the infinitive. The object pronoun(s) may never, ever be placed between the auxiliary and the complementary infinitive. When placed on the end of the infinitive, a written accent must be placed over the syllable that naturally receives the stress when no objects are added.
Él te lo quiere dar./Él no te lo quiere dar.
Él quiere dártelo./Él no quiere dártelo.
When the progressive is used (any tense of the progressive; here I show only the present progressive), likewise, the object pronoun(s) may be placed either in front of the conjugated verb (usually estar) or attached to the end of the gerund.
As before, the object pronouns may never, ever be placed between the verb estar and the gerund. And, as before, if the object pronoun(s) are placed on the end of the gerund, a written accent must be placed on the syllable that naturally receives the stress when no object or objects are added.
Él te lo está dando./Él no te lo está dando.
Él está dándotelo./Él no está dándotelo.
This last scenario applies only to commands, or imperatives. Unlike the second and third scenarios, there are no options regarding placement of object pronouns when using imperative verb forms. The placement depends entirely on whether or not the command is affirmative or negative. The following examples use Ud. commands, but the placement rules apply to all command forms.
Note that the negative command follows the pattern established by scenario one: the object pronoun(s) must precede the verb. When the affirmative command is used, the object pronoun(s) must follow and be attached to the imperative form of the verb and the written accent placed on the originally stressed syllable of the imperative.
¡No se lo dé!