The infinitive is the most basic form of a verb. It is composed of the verb stem and the ending "en" or "n". You've already seen this form when you learned about the modal verbs.

Example:
Harry soll sein Zimmer putzen.

You'll often see the infinitive form in connection with the word "zu". This is called an infinitive construction that is usually accompanied by other words or groups of words. These constructions refer back to a word in the independent clause. They have no conjugated verb or subject of their own. The subject derives from the independent clause. The "zu" + infinitive verb comes at the end of the subordinate clause and the two words are usually written separately.

Example:
Harry Walkott hat keine Lust, das Zimmer zu mieten.
Heute ist Tom an der Reihe, das Geschirr zu spülen.

Not all main and dependent clauses can be given an infinitive construction. There are certain expressions that make the additional phrase possible, for example:

1. "haben" + noun
Ich habe kein Interesse, das Zimmer zu mieten.
Tom hat die Aufgabe, das Bad zu putzen.

2. Impersonal expressions
Es ist wichtig, das Experiment zu stoppen.
Es ist schön, in Deutschland zu sein.

3. The verb "helfen"
Ich will helfen, das Experiment zu stoppen.

Using infinitive constructions with separable verbs
Some verbs have a prefix that can be separated from the conjugated form of the verb. These are called separable verbs.

To use the infinitive construction with separable verbs, the "zu" comes between the verb and its prefix and all the elements - prefix + "zu" + infinitive - are written together as one word.

Examples:
ein|kaufen:
Tom kauft ein. – Tom hat die Aufgabe, einzukaufen.

runter|bringen:
Tom bringt den Müll nicht runter. – Tom hat kein Interesse, den Müll runterzubringen.

Comma placement
The infinitive phrase is often separated from the independent clause by a comma. But that's not mandatory. You can leave it out if the infinitive construction is especially short.

Examples:
eher ohne Komma: Harry hilft(,) zu putzen. (= better without a comma)
eher mit Komma: Harry hilft, das Bad zu putzen. (= better with a comma)