How to pronounce v and f in English
Every time I take the bus in Munich, I hear the number five (as in U5, the underground line I take) being pronounced incorrectly, namely as [faɪf] instead of [faɪv]. You can hear the correct pronunciation here. This common mistake among German speakers is also often made in the case of live; live is correctly pronounced as [laɪv], whereas life is pronounced as [laɪf]. Listen to the two different pronunciations here and here. (Note that the adjective live is pronounced differently than the verb to live, see here.)
The pronunciation [laif] for live is acceptable when speaking German, see here. However, in English it is wrong. In the case of five, there is no German pronunciation, so it should always pronounced as [faɪv].
A confusion of the pronunciation of the letters f and v is very common among German speakers trying to learn English. For example, view is often pronounced like few, and serve is often pronounced like surf, even though these words are not homophones and have rather distinct correct pronunciations. As a basic rule, remember that the letter v is never pronounced like f in English (whereas it is almost always pronounced like f in German). Hence, every is pronounced as [ˈevri] not [ˈefri], give is pronounced as [ɡɪv] not [ɡɪf], and have is pronounced as [həv] not [həf].