Verbs that have two parts

In a Spanish sentence, the usual place for a direct object pronoun is before the verb. (If you haven't learned that yet, visit the page on direct object pronouns.) But there are some exceptions to that rule. Two of those exceptions are the progressive tenses and two-verb constructions that use infinitives (for example: Estoy comiendo pizza. Estábamos pintando la casa. ¿Quieres estudiar?) You might notice that they consist of more than one part-the conjugated part (like quieres or estoy) and the invariable part (a present participle like comiendo or an infinitive like estudiar). The other thing that they have in common is that when you use a direct object pronoun with them, the direct object pronoun may be placed before the conjugated part of the verb or after the present participle (that's what -ing words are called) or the infinitive. Here are some examples.

¿La pizza? La estoy comiendo.	¿La pizza? Estoy comiéndola.
¿A mis amigos? Los quiero invitar.	¿A mis amigos? Quiero invitarlos.
¿Las tareas? No las voy a preparar.	¿Las tareas? No voy a prepararlas.

When you add a pronoun to the end of a present participle (-iendo and -ando), you have to add an accent. One of the rules of written accents in Spanish is that when the tonic stress (the spoken stress: like ha-BLAR or can-TAN-do) falls three or more syllables from the end of the word, you need to place a written accent on the vowel in that syllable. You may remember that rule if you like, or you may simply remember that when you add a direct (or indirect or reflexive) pronoun to the end of a present participle, you will need to write an accent on the a of ando or the e of iendo. It doesn't matter whether you place direct object pronouns at the beginning or the end of these constructions; there is no difference in meaning. Choose the method that makes most sense to you and learn to use it. Be aware of the other method, too. You will certainly hear and see both methods in the language of native speakers of Spanish.

Affirmative commands

Affirmative commands are different. Spanish requires that direct object pronouns be attached to the end of affirmative commands, as you will see in these examples:

¡Come la pizza!	¡Cómela!
¡Inviten ustedes a mis amigos!	¡Invítenlos ustedes!

If you take a close look at the examples on the right, you will notice an accent mark. It's the same rule of accents that you read about above: when the tonic stress is three or more syllables from the end, add a written accent mark. If you have a hard time hearing where the tonic stress is in command forms, you will usually find on the vowel that is in the syllable right before the verb's ending (COM-e, in-VI-ten, al-MUER-ce).