I have begun to look through the Grammar charts and word entry's in "A
Concise Dictionary of Old Icelandic" by Geir T. Zoëga but have become
confused with the syntax of the entries. For example, the entry for
'ormr':

ormr (-s, -ar), m. snake, serpent.

So, I would assume that this would then belong to the "Strong
Masculine 1st Declension" group, because that is titled with "1st
Declension, gen. sing. -s, nom. pl. -ar, acc. pl. -a."

I do not understand why Zoëga did not then just list it as

ormr (1st), m. snake, serpent

But I digress. The entry for 'fors':

fors (pl. -ar), m. waterfall.

Now, which declension is one to assume this noun belongs to?

As well, are nouns whose entries are listed simply as

noun, gender. definition.

in the "weak declension" category?

What is more is that there seems to be no explanation for
abbreviations, such as "pr." and "pp." or "p."

Also, charts for weak/strong verbs and their classes/declensions are
given, but how does the entry of a verb indicate which it belongs to?

So it seems my stream of questions is over for the moment! :)

-Sebastian