Think about "another" as just "an+other". If the noun being modified or referred to is singular, then use "another".

Jose Antonio, you would not believe how often this question comes up here. I could swear that there are some weeks when there's is a Spanish speaker asking this question every day! We also once had a discussion thread in which someone asked 'If you could change one thing about your language, what would it be?'. My first answer, was I wish that English would go back to spelling 'another' as two words - 'an other'. Because it would save learners, and their teachers, so much time and effort if we simply wrote it as two words: One other person = an other person = another person Honestly, it is that simple! Another just means 'one other'. And to answer your comment. Yes, you can say 'I have another book'. But remember that 'other' is an adjective, and English adjectives are invariable, so it's the same in the plural: 'I have other friends'.