Actually, one book on this, is much like another.

Internal conflicts, called 'subplots,' is a difficult thing to teach.

To summarize the external & internal conflicts, which we will call 'subplots is more simple:

You, the writer, created the characters. In order to make them '3-D' and alive, they will have to live their lives.

Life throws unexpected things at us, and we all deal with those 'things' differently according to our character and the way we were raised, and other external and internal factors.

A character that is of ill-repute, having no morals and principals, (a bad guy) might deal with a conflict of loosing his mother, by going out on a rampage and getting drunk, and killing people.

On the other hand, a character of good moral (your good guy) might have a lot of internal dialogue with himself, seek solace in a friend ( a female companion-friend ) and cry, maybe get drunk when he normally doesn't drink, have a car accident and almost die, in order to deal with the death of his mother.

These are internal & external conflicts.

Internal is how they deal with things and issues they have, in making decisions or worries, etc. and external conflicts are things that happen that's out of their control, and they have to deal (react) on them.

Life is the drama via subplots- it can be anything- but it happens to the characters as they evolve and develop and lead their lives in your book and within your main plot- they create the subplots so the reader can root and cheer for them, and the reader can think, 'I hope he gets what he deserves' concerning the bad guy(s), and ,' I hope he's okay and does this...' or, ' he can get through this-- the author would'nt let anything bad happen to HIM!' concerning the good guy(s).

Those are very 'superficial' conflicts to make an example.

Your conflicts and dynamics should be believable and serious, engaging and interactive, and cause your reader to identify and root and cheer for the outcome-- each character deals with conflicts in his own unique way, according to his character and personality.

These subplots are what makes your characters alive and '3-D' making them believable as in 'real.'