What is the difference between rin (din) and pa rin?
I understand that rin turns to din when the preceding word ends in a vowel. Likewise, raw to daw, etc. That is not my question.
I often see the word rin used is a sentence and understand it to mean also. But I also see pa rin used in a sentence. Since pa translates to "still", "yet" it is not clear to me what pa rin (still also) implies? It seems that there is no use for the word pa. Please explain.

Question

What is the difference between rin (din) and pa rin?
I understand that rin turns to din when the preceding word ends in a vowel. Likewise, raw to daw, etc. That is not my question. 
I often see the word rin used is a sentence and understand it to mean also. But I also see pa rin used in a sentence. Since pa translates to "still", "yet" it is not clear to me what pa rin (still also) implies? It seems that there is no use for the word pa. Please explain.

Answer

You are right with "pa" translating to "still" or "yet". Example:
"Hindi pa" = "Not yet"

"rin" or "din" is used to add weight to the expression. If you recall the meaning, "rin" or "din" is used to express similarity/sameness and usually translates to "too".

For e.g. "Your book is big too"  = "Your book is similarly big" = "Ang libro mo ay malaki din/rin"

In the case of "pa rin", it interprets to mean that something is still the same or some event is still in progress BUT this has more weight that it expresses a slight level of frustration.

"Pa rin" is usually used in circumstances where a person is expressing that something has been still the same for a duration that is longer than expected.

E.g. This example is when things are still the same within the duration that's anticipated.
Q. "Are you done yet?"/ "Tapos ka na ba?"
A. "Not yet" / "Hindi pa"

Say, a duration longer than expected has passed and a person asks you again the same question, the following are applicable:
Q. "Are you done yet?"/ "Tapos ka na ba?"
1. "Not yet"/ "Hindi pa"
That is fine because it is grammatically correct.
BUT
2. To add weight to the expression, considering that a moment has already passed beyond one's expectation you may add a "rin/din"
"Hindi pa din" / "Hindi pa rin"

"rin" or "din" may look grammatically irrelevant when added to "pa" but expression-wise you will find the difference.

Hope that helps! :)

Answer

"pa" may exaggerate and intensify expressions :

'ako ang nagwagi' ("I am the champion" - implying "I have beaten everyone")
'ako PA rin ang nagwagi' ("I am still the champion" - implying "no one still can beat me")

"pa" as a particle, can also be equalled to "more" in English.

'isa pa!' ("one more!)
'kain ka pa' ("eat more")

in fact, there are many ways you can use "pa".

anticipation: "malayo pa ba tayo (sa pupuntahan natin)?" > are we there yet?

:)

Answer

We, Filipinos, use pa in different ways.
example:
Wala rin akong asawa means I have no husband/wife like you. (It's comparing your situation with your listener/reader)
Wala pa rin akong asawa hanggang ngayon. pa is commonly used with hanggang  ngayon. It means I still have no husband/wife up to now. (It's more emphatic in Filipino.)
Wala pa akong asawa means I still have no husband/wife. (You're just plainly stating it.)