Some rules of thumb:
1) A normal short or unstressed A (phonetically a short "ɐ" or "ä") is always used at the end of plurals, examples:
BOKA, THINGA, GÀRSA, BÔMA, MANGÉRTA, MÀNNESKA, FRÜCHDA, NOCHTA, DRÁMA, SVNA, TOGHATERA. (Note: some of these plurals also occur as ending with -UM in stead of -A.)
2) Infinitives of verbs also end with -A, examples:
WÁRJA, -DÀRVA, -LÍSA, -SKRÍVA, KVMA, WÉSA.
Also conjugations ending -ATH, -ADE, -ANDE, examples:
-BIFATTATH, -BINOMATH, MÁKADE, GLÍANDE.
3) The accentuated open A (phonetically "a") is spelled Á, examples:
WÁRJA, ÁK, THÁ, ÁT.LAND, WÁK, MÁKADE, DÁNA, KWÁD, BLÁT, ÁDAMA, DRÁMA.
The modern Dutch varieties of these words are often spelled/pronounced with long A or long O (ook, waak, maakte, daarna, kwaad, bloot, adem, dromen), German AU or O (auch, Traum/Träume, bloß)
4) The À (phonetically "æ" or "ɛ"; as in Englis "that") is often interchangeable with E:
ÀFTER / EFTER, -DÀRVA / -DERVA, BÀRN / BERN, THÀT / THET, GÀRS / GERS.
5) Many of the plain A's (mostly at the end of a word) have disappeared in our modern languages, indicating that they were probably not pronounced clearly, examples:
BOKA === (D/F) boeken (G) Bücher (E) books
KVMA === (D) komen, (F) komme, (G) kommen, (E) come
THISSA === (D) deze, (F) dizze, (G) diese, (E) these
VSA === (D) onze, (F) ús, (G) unsere, (E) our
THRJA === (D) drie, (G) drei, (E) three
NAVT === (D) niet, (F) net, (G) nicht, (E) not
and: THINRA, HJA, RA
6) Some words still exist more or less in the same form, examples:
AL- (as in ALLE, ALLET, ALHWAT) === (D/F/G) al-, (E) all-
AS === (D/G) als, (F/E) as
ANFANG === (D) aanvang, (G) Anfang