Normally, when you form the Perfekt tense, you use a conjugated helper verb (haben or sein) alongside a past participle (like gespielt or gesehen).
However, there is a special rule for certain non-modal verbs when they are used alongside another action verb. The most common of these are sehen (to see), hören (to hear), lassen (to let), and helfen (to help).
When you use these in the Perfekt alongside another verb, they do not take a past participle. Instead, both verbs remain in their infinitive forms, stacked at the very end of the sentence. This is known as a double infinitive.
Seeing and Letting: sehen and lassen
To say you saw someone do something, place the action verb's infinitive followed immediately by sehen at the end of the sentence.
| I saw the toddler run onto the playground without supervision. | Ich habe das Kleinkind ohne Aufsicht auf den Spielplatz laufen sehen. |
Notice how the sentence ends with laufen sehen rather than laufen gesehen.
The verb lassen works exactly the same way when you let or allow someone to do something.
| On the playground, we let the careless girl swing. | Auf dem Spielplatz haben wir das unvorsichtige Mädchen schaukeln lassen. |
Helping: helfen
You also use the double infinitive with helfen. Remember that the person receiving the help must be in the dative case (dem Mädchen).
| I helped the girl build in the sandbox. | Ich habe dem Mädchen im Sandkasten bauen helfen. |
Hearing and Multiple Actions: hören
The verb hören also triggers this rule. If you hear someone doing more than one thing, you simply list all of the action infinitives right before hören.
| We heard the children laugh and hop on the playground. | Wir haben die Kinder auf dem Spielplatz lachen und hüpfen hören. |