The infinitive is the basic form of a verb. If you don't know a verb and look it up in a dictionary, the dictionary gives you the infinitive, or more precisely the plain/bare present infinitive of the verb in question:
speak, know, need etc. The full infinitive has the particle
to before it:
to speak, to know, to need. In English, the infinitive can
never be inflected, which means that you cannot add an ending to it or conjugate it in any way. Examples of present
infinitives:
I want to speak English. You must speak English. We will speak English. I had an opportunity to speak English. It's easy to speak English. They made us speak English. You had better speak English.
There are (only) two infinitives in English. Besides the above present infinitive, there is the perfect infinitive, which consists of [to] + have + past participle. It often refers to the past and is uninflected like the present infinitive. Examples of perfect infinitives:
He may have spoken English there. I wouldn't have spoken English. They need not have spoken English.
Both infinitives can be used in the passive voice as well and in this case you indeed have two past participles next to each other (have been + past participle). Examples of the present infinitive in the passive (be + past participle):
English must be spoken here. He wanted to be taken into account. She didn't expect to be seen there.
Examples of the perfect infinitive in the passive:
He may have been seen there. Something should have been done. To have been seen isn't the same as to have been caught.
You can read more about infinitives here.
English infinitives are very simple compared with the infintives of some other languages. You'll soon learn them. 
The Germanic languages are a group of closely related languages including English, German, Dutch, Swedish etc. You can read more about them here.
You have a computer and the Internet at your disposal. You should be able to find out about basic things like these on your own.
CB