In short

Lesson 30 committed you to enclisis: in a plain affirmative clause the pronoun hangs after the verb, tied on with a hyphen. That is fine for me, te, se, lhe - they never change. But the direct-object clitics o, a, os, as are fussy about what sits in front of them. Attach one to a verb form ending in -r, -s or -z, and that consonant drops and the pronoun turns into -lo, -la, -los, -las: comprar + o -> comprá-lo, fez + o -> fê-lo. Attach one to a verb ending in a nasal sound (-am, -em, -ão, -õe), and the pronoun grows an n: -no, -na, -nos, -nas - compram + o -> compram-no, põe + a -> põe-na. This is not a new pronoun and not a new rule of placement - it is a spelling reshaping at the join, and once you see the two triggers you can produce it on the spot. It only ever happens in enclisis; the moment a proclisis trigger from Lesson 30 pulls the pronoun in front, the plain o/a comes straight back.

The rule

One trigger: the last sound of the verb

Before you attach o, a, os or as, look at how the verb form ends. That single fact decides the shape:

• ends in -r, -s, -z -> the consonant drops, clitic becomes -lo / -la / -los / -las;
• ends in a nasal (-m, or the nasal diphthongs -ão, -õe, -ãe) -> clitic becomes -no / -na / -nos / -nas;
• ends in a plain vowel -> nothing changes, keep -o / -a / -os / -as (dá-o, vê-a).

The direct clitic still carries the gender and number of the noun it replaces, exactly as in Lesson 29 - only its outer shape is reshaped.

-lo/-la after -r, -s, -z

The final consonant of the verb is deleted and the clitic gains an l-. When the deletion leaves the verb ending in a stressed -a, -e or -o, that vowel takes a written accent (a stressed -i stays plain):

• After -r (infinitives) - the workhorse case, because the pronoun so often lands on an infinitive (vou…, quero…, para…). An -ar stem takes an acute á, an -er stem a circumflex ê, an -ir stem keeps a plain stressed -i:
  • comprar + o -> comprá-lo, falar + as -> falá-las
  • comer + a -> comê-la, vender + os -> vendê-los, pôr + o -> pô-lo
  • abrir + a -> abri-la, conduzir + o -> conduzi-lo
• After -s - most often a nós form in -mos, whose -s drops (the same drop you already make in sentámo-nos, Lesson 12): comprámos + os -> comprámo-los, fazemos + o -> fazemo-lo, pusemos + a -> pusemo-la.
• After -z - a third-person or imperative form in -z: faz + o -> fá-lo, fez + o -> fê-lo, diz + a -> di-la, traz + os -> trá-los.

-no/-na after a nasal ending

Here the verb keeps its full spelling and it is the clitic that changes, gaining an n-. This is the easier of the two: add nothing to the verb, just start the pronoun with n:

• 3rd person plural in -am / -em / -ão: compram + o -> compram-no, fazem + a -> fazem-na, dão + o -> dão-no, viram + a -> viram-na, têm + o -> têm-no.
• Singular forms ending in a nasal diphthong -ão / -õe / -ãe, or in -m: põe + a -> põe-na, tem + a -> tem-na.

It only happens in enclisis

These allomorphs are purely a join effect. Nothing reshapes when the clitic is not glued to the end of the verb. So the instant a proclisis trigger from Lesson 30 appears - negation, a conjunction, a question word, an adverb like já / também / só - the pronoun jumps in front and reverts to the plain form:

• fá-lo -> não o faz / quem o fez?
• comprá-las -> já as comprei / que as compres
• compram-no -> nunca o compram

Forms & patterns

The two reshapings side by side:

The infinitive accent, by conjugation (the pattern you will use most):

Examples

Common mistakes

• quero comer-o -> quero comê-lo - after an infinitive’s -r the pronoun is never plain -o; the -r drops and the clitic becomes -lo (with an accent on the stem, here ê).
• vou fazer-lo -> vou fazê-lo - you cannot keep the -r and add -lo; the whole point is that the -r disappears (fazer -> fazê-lo).
• comê-la written comela or come-la -> comê-la - the reshaping is not optional and the stem accent is part of the spelling: -ar -> á, -er -> ê.
• compram-o / dão-o -> compram-no / dão-no - after a nasal ending the clitic must take the -no/-na shape; plain -o after -m or -ão is wrong.
• comprámos-los / comprámos-os -> comprámo-los - the -s of the nós ending drops first, then -los attaches: comprámo-los.
• faz-o / fê-o -> fá-lo / fê-lo - a form ending in -z drops it and takes -lo; faz + o is fá-lo, fez + o is fê-lo.
• não fá-lo / que compram-no -> não o faz / que o compram - the allomorphs live only in enclisis; once a trigger forces proclisis, the plain o/a returns in front of the verb.

Tips

• Decide the shape from the verb’s last sound before you attach: -r/-s/-z -> -lo; nasal (-m/-ão/-õe) -> -no; plain vowel -> plain -o. That one check settles every case.
• The -lo side does two things at once - drop the consonant and (for -ar/-er infinitives) accent the stem vowel: falá-lo, vendê-lo. The -ir verbs are gentler: just a plain -i, as in abri-lo.
• The -no side is the easy one: change nothing on the verb, simply start the pronoun with n-. If the verb ends in -m or a nasal -ão/-õe, reach for -no/-na.
• Say the result as a single word - co-mê-lo, com-pram-no - and you will hear why the reshaping exists: it just makes the join pronounceable. It is spelling following sound, not a rule to memorise cold.
• When you are unsure, remember you can always dodge: rephrase with a proclisis trigger (não o compro) or repeat the noun (compro o pão), and the plain o/a comes back untouched.
• In Brazil: these enclitic -lo/-no forms survive in careful writing but are rare in speech - Brazilians lean on proclisis or simply drop the third-person object (vou comprar, já comprei), so comprá-lo and compram-no sound markedly formal there.