Maybe you have seen in some sentences that there are times when we use only infinitiv of verbs and sometimes we use zu+ infinitiv structure. How do we determine which option to use?

It is important to see if we have the subject in the sentence where we want to put that verb. This always happens in sentences that have more than one simple sentence - meaning, more than one predicate; sentences that are made of couple of sentences where one of them depends on the other. If we have subject in that particular sentence (part of the bigger sentence), then we should never add "zu", it would be completely wrong. Also, we can notice that when we have modal verb that should be "completed" with some verb, we only write infinitive, never add zu there.

Here are some examples:

Ich denke, dass wir nach Hause gehen sollen. -----> I think that we should go home.

Es ist eine gute Idee, nach Hause zu gehen. ------> It is a good idea to go home.

Ich bin sicher, dass ich das gut machen kann. -----> I am sure that I can do it well.

Ich denke, dass es spät ist, das zu machen. -----> I think that it is late to do that.

There is also a rule that every sentence in which we used zu+infinitiv can be rewritten as a sentence with only infinitiv, by just adding subject to the sentence. Let's see that on our examples:

Es ist eine gute Idee, nach Hause zu gehen. ------> Es ist eine gute Idee, dass wir/ich nach Hause gehen/gehe.

It is a good idea, to go home. -------> It is a good idea that I/we go home.

Ich denke, dass es spät ist, das zu machen. ----> Ich denke, dass es spät ist, dass ich/wir das mache/machen.

I think that it is late to do that. --------> I think that it is late that I/we do that.

The structure with zu is easier to say, understand and is more commonly used, but it is good to know how to rewrite it in this longer form. That way you can see, if you can't rewrite it, then you shouldn't add zu, it shouldn't be in that sentence.

If you have some examples or maybe some doubts about if you should put zu in some concrete sentence, feel free to leave it in the comments!