Importing text from Word to ID

Hello. Which are the best way to import text files? I mean, step by step, from Microsoft Word to ID? It is a headache when you apply an style and booom all the italics and bolds dissapear... Sometimes it works fine when i apply a style, but sometimes it does a disaster. From now, I am really curious about how indesign thinks to resolve the import cases and the work of styles.

1) Why sometimes erase all the word local formats and sometimes does not?

2) Is it a problem using .doc, .docx or .rtf to save text files an import?

3) Is the problem that you work with .TTF fonts (like Arial, Times, Courier) from word and on ID you use OpenType Fonts?

4) What is the NON-FAIL way to do an excellent import? from Microsoft Word to ID

Thanks

Alejandro