It becomes clearer, if you look at complete sentences, not at just a few words:
- Ich werde das Auto verkaufen.
- Das Auto ist zu verkaufen.
In English:
- I will sell the car.
- The car is for sale.
#1
In #1 we have a normal statement that describes an action that will happen in the future. You can transform this sentence into present tense, which makes the infinite form to be replaced by a finite, i.e. declined form:
Ich verkaufe das Auto.
I sell the car.
In German future tense is build by a form of the auxiliary verb werden, that has to be declined according to the subject (which is ich, i.e. 1st person singular) plus the main verb in its undeclined infinitive form. The finite (i.e. declined) part (»werde«) has to stand on position 2 of the sentence, while all infinite (i.e. not declined) parts have to stand at the end of the sentence.
#2
In #2 we have a sentence in present tense, that doesn't describe an action. It describes a state. This sentence has no object. It just has a subject (das Auto) and a predicate (ist zu verkaufen). The predicate is of the form
(form of haben or sein) + zu + (infinitive of the main verb)
Examples for this construction are:
Ich habe zu tun. = I am busy. (verbatim: I have to do)
Das ist zu schaffen. = We can do/make it. (verbatim: This is to make/create/manage)
The verbatim translation for
Das Auto ist zu verkaufen.
is
The car is to sell.
But you don't use this construction in English, so you say instead »The car is for sale«.
This also works in ellipsis (incomplete sentences). In both languages you omit the article of the subject (the, das) and the auxiliary verb (is, ist). I put this words into subscript:
The Car is for sale in Berlin.
Das Auto ist in Berlin zu verkaufen.
Note the word order! The infinite parts of the predicate come direct after the finite part in English, but at the end of the sentence in German.