"sage" is the du-imperative form, and I don't see "sag" on any "sagen" conjugation pages.
There are several ways to convey a command or request in German.
The imperative proper is derived from the respective 2nd person present indicative form, which may differ from the infinitive stem (see sprechen below). It has no suffix for a singular addressee and a +t for plural addressees. When the +st and +t suffixes require an e in front it should also appear in the imperative, in some cases it even must appear.
An alternative for the singular, which is not covered by every grammar book or German course, because it’s still not considered standard German, is the simpler and less strict, more polite form with e in all cases. It’s just the infinitive without the n, so basically identical to 1st person singular indicative and 1st or 3rd person singular subjunctive. Like most final schwas in German, this one may be left unspoken, too, and could then be rendered by an apostrophe in writing. This would result in the same form as standard imperative for regular verbs (sag’!), but not for “strong” ones (sprech’!).
Since the polite 2nd person singular pronoun Sie is equivalent to the 3rd person plural sie, the imperative is the same form. The 1st person plural can only have an imperative (like let’s in English) and it has (like always) the same form as the 3rd. The only caveat is that the pronoun (after the imperative) is always mandatory for these persons, whereas it’s impossible for the others unless a more complete phrase follows! (It was possible when peasants were addressed with the 3rd person, e.g. spreche Sie! / sprich Er!.)
- sag! < du sag+st
sag+t! < ihr sag+t
sag+e! < ich sag+e, sag+en
sag+en wir/Sie! < wir/sie sag+en
- sprich! < du sprich+st
sprech+t! < ihr sprech+t
sprech+e! < ich sprech+e, sprech+en
sprech+en wir/Sie! < wir/sie sprech+en
- rede! / red! < du rede+st
rede+t! < ihr rede+t
red+e < ich red+e, red+en
red+en wir/Sie! < wir/sie red+en
- atme! < du atme+st
atme+t! < ihr atme+t
atm+e < ich atm+e, atm+en
atm+en wir/Sie! < wir/sie atm+en
- kicher! < du kicher+st
kicher+t! < ihr kicher+t
kicher+e! / kich’re! < ich kicher+e / kich’re, kicher+n
kicher+n wir/Sie! < wir/sie kicher+n
- näsel! < du näsel+st
näsel+t! < ihr näsel+t
näsel+e / näsl+e! < ich näsel+e / näsl+e, näsel+n
näsel+n wir/Sie! < wir/sie näsel+n
For the record, there is no direct imperative for the 3rd persons and the 1st person singular. Phrases with mögen or sollen are possible.
The way I learned the imperative was that you take the conjugation of the verb for the appropriate case, and then drop the respective ending (eg,
-st for the du form).
By this formulation, we have
sagen -> sagst -> sag.
This has nothing specifically to do with the imperative. Any inflected form ending in an unstressed -e is liable to lose the final e, whether it's an imperative ("schreibe!"), a 1st person form ("gehe"), a dative ("Lande"), or a plural ("Schuhe"). Not all of these simplifications are equally far along, but all are widespread.
Once a trend like this has advanced sufficiently, grammarians will retrofit it into the inflection table so that the short form is standard and the long form archaic, but the impetus to do so is a general phonological one and not one of grammar.