When these expressions are followed by another phrase, think of "ewan ko" as "I don't know about". So, “Ewan ko ang pangalan niya” becomes “I don't know about his/her name.” As is, that won't sound right though. But if that becomes part of a longer sentence, e.g., "My name was already called, but I don't know about his/her name", it will be all right. Consequently, with the "about" in there, you should now use "ewan ko" and not "hindi ko alam" for its Tagalog translation - "Natawag/Tinawag na ang pangalan ko pero ewan ko ang pangalan niya".
Quite often, these expressions are used before the word “kung”. “Kung” normally means “if/whether”. When followed by question words – ano (what), sino (who), kailan (when), bakit (why), paano (how), alin (which) – “kung” may be interpreted as “as to”.
Hindi ko alam kung ano ang pangalan niya. = I don't know (as to) what is his/her name.
Ewan ko kung ano ang pangalan niya. = I don’t know about that which is his/her name.
In the last example, “kung ano” was translated to “that which” to allow the “I don’t know about” (ewan ko) to precede it. However, as a rule, either “hindi ko alam” or “ewan ko” may be used if the phrase immediately following it begins with “kung”.
Hindi ko alam/Ewan ko kung bakit. = I don’t know as to why.
Hindi ko alam/Ewan ko kung alin. = I don’t know as to which one.
Hindi ko alam/Ewan ko kung matutuwa ka. = I don’t know if you’ll be happy (about it).
Hindi ko alam/Ewan ko kung matutuwa ka o hindi. = I don’t know whether you’ll be happy or not (about it).