So... to summarise then:
Мне надо сегодня помочь матери с домашними делами.
right
Он хочет изучать русский язык в университете.
right
Мне надо кормить собаку три раз в
день (
verb is correct)
Мне надо теперь накор
мить кошку.
aspect is right
So it's basically dependant on whether the action performed by the verb is a one off, or not? Or the duration of the action?
Basically, yes. But I think there might me some other nuances as well.
I found another example that doesn't seem to conform to the rule, though. What happens here?
Я всегда рад помочь тебе - Surely the word "always" in this sentence implies that the action could take place many times... so why is the infinitive in this sentence perfective?
Я люблю помогать друзьям. - the rule works here
Я рад помочь другу. Я рад прочитать новую книгу. Я всегда рад посмотреть интересный фильм.
It seems to me that the word рад just requires prefective verbs.
There are some cases when the aspect of an infinitive is defined by the main word (рад in this case). At least, when рад is used figuratively, in the sense of "is ready to do someting".
However, there are such cases as:
Я всегда готов читать книги.
I am always ready for reading books. It means, I am always ready to spend time reading.
and
Я всегда готов прочитать интересную книгу.
I am always ready to read an interesting book. It concentrates attention not on the process of reading, but on the result (to have a book read, from beginning to the end).
I am not sure I covered all the cases here, thus, ask questions ![](http://masterrussian.net/images/smilies/KOLOBOK/smile.gif)
Спасибо за помощь.
Джек