English words are difficult to read and to write correctly. The vowel letter 'a' may be used to represent many 'vowel sounds'.

22 - a e i o u (y) : many true sounds of 'a' (PSE)

We have learned about vowels (เสียงแท้; สระ - สะ หระ).

In Thai, there are more sounds than the number of alphabets representing vowels. In English, we have learned that there are only 5 vowels ( a e i o u ) and a special alphabet 'y' [which can be consonant (พยัญชนะ; กลมกลืน) and vowel] and a lot more sounds.

We know that a word is made up from the sound of a vowel modified by the sounds of consonants. For examples: at = แ-ะ + ทึ, on = ออ + นึ, go = กึ + โอ

In Thai words, all vowels sound exactly as they are written (but in some Pali/Sanskrit words สระ 'ะ' may be implied but omitted, and consonant clusters { อักษรควบไม่แท้: จร, ซร, ทร, สร, ศร } can cause confusion in reading and writing Thai
.)   
As we have heard, English words are difficult to read and to write correctly. The vowel letter 'a' may be used to represent many 'vowel sounds'. For examples:

เอ    a (the letter a for apple), any, baby, bacon, bay, bake, cake, came, clay, crater, day, drake, fake, fame, game, gay, hay, jay-walk, lake, lame, lane, late, make, mane, may, name, nape, pay, pane, plane, race, rake, rate, sane, sway, tame, tray, stay, wake, way, vane, ...   

แอะ    apple, back, bat, cap, cat, chat, crack, fat, fact, gap, gnat, hack, hap, hat, has, map, mat, nab, nap, pack, pat, rack, rap, rat, sack, sap, sat, tack, tap, tat, vat, wack, ...
 
แอ    bad, bag, bamboo, ban, bare, caddy, caggy, camera, camp, care, can, dam, candy, dandy, dare, fad, fag, fan, fare, gag, hag, lag, (slag), lamb, lamp, mad, mag, man, mare, nag, (nomad, gonad), snare, ram, ran, snare, pad, pan, panel, pare, pram, spare, sad, tad, van, wad, ware, ...

อะ    about, above, abrupt, account, accuse, admire, afar (อะฟาร์), affect, ajar(อะจาร์), alike, alpha, amaze, apply, acquire, arise, asunder, at, attest, attire, attend, awake (อะเวค), aware, away, awhile, baboon, canoe (คะนู), (kayak คะยัค), calculate, canopy, ... (many many more)

อา    bar, barber, barometer, Calorie, car, carpet, darling, far, father (ฟาเดอรึ), (mother มาเดอรึ,) garden, harrow, fallow, Java, javelin, mal-formed, malfunction, marble, march, mark, market, marlin, marrow, marsh, narrow, parrot, quary, ravine, saloon, talon, tart,  vast, ... (a lot more)

ออะ/ออ    all, ball, call, mall, malt, stall, tall, wall, waltz, ...

ออ    brawl, claw, dawn, draw, fawn, gawk, haw, hawk, hawker, jaw, paw, pawn, prawn, raw, saw, sawn, taw, tawny, war, warm, was, wash, watch, what, ...

เออะ    'a' (article before a noun eg. a man
     we say เออะ แมน in normal use of article "a";
    we may say อะ แมน to mean "just one" man;
    we may say เอ แมน to mean "any one" man.)

We can see 'a' is used to represent many different vowel sounds in many different words. If we read the words incorrectly (or make different sounds), listeners would not 'get' (understand) the words or the meanings. For examples: reading 'claw' as 'คลาว' instead of 'คลอวึ'; reading 'pan' as 'เพน' instead of 'แพน', and so on.

It is difficult for Thai learners to read English because English vowels are not represented 'one letter-to-one sound'.

Is there any rule? There are patterns that we may recognize, for examples 'a' followed by 'e' or 'y' usually reads as 'เอ'; 'a' followed by 'r' reads as 'อา'; 'a' followed by 'w' reads as 'ออ' but 'war' reads 'วอร์' not 'วาร์'. So, there are exceptions. In facts there are more exceptions that make learning by rules more difficult than learning by 'senses' or memory.

Nevertheless, we can see that 'a' more often sounds like อะ or อา (in UK) แอะ or แอ in US.

In Australia (อ็อสสะ เตร เลีย in Thai) sounds อ็อสสะ ตรา  เลีย ;-)
 
[More to come.
You can help by giving more examples of words with letter 'a' and how (vowel) they read.

Remember? Learning PSE is about NOT waiting to receive instructions or knowledge. It is about going out to get new objects, new experiences, new ways for learning and new friends to enhance our learning iterations (by using and modifying our 'senses'). Each time we do a round (loop), we should also test for correctness. If we do not know, we should ask (the right questions) straight-away.]

How do we read names like:
   Abel, Abigail, Aaron, Hasan, Ayuddhaya,
   Sarah, Samantha, Vera, Tara, Jared, Maria, Nathan
   Abraham, Sam, gramp (grandfather), granny (grandmother), ...

Reference for Thai sounds:
นายอับดุลเลาะ แขวง [ครูภาษาไทย], "ระบบเสียงในภาษาไทย", โรงเรียนบ้านกาตอง ตำบลกาตอง อำเภอยะหา จังหวัดยะลา
http://eduvc.oas.psu.ac.th/~user168/intro.html

http://www.dutjai.de/PT009.html
อักษรควบไม่แท้ คือ พยัญชนะที่มีตัว ร ควบอยู่ด้วย แต่ไม่ออกเสียงตัว ร หรือมิฉะนั้นก็ออกเสียงเป็นเสียงอื่นไป

อักษรควบไม่แท้ที่ออกเสียงแต่พยัญชนะตัวแรก ได้แก่

จร    ตัวอย่าง        จริง                
ซร    ตัวอย่าง        ไซร์                
สร    ตัวอย่าง        สรง    สร้าง    เสริม    สระ    สร่าง
ศร    ตัวอย่าง        ศรี    เศร้า    เศรษฐี        

อักษรควบไม่แท้ ทร ออกเสียงเป็น ซ ได้แก่  ไทร   ทรง   ทราบ   ทราม   ทราย  ทรุด   โทรม   อินทรี   อินทรีย์   นนทรี   ทรัพย์   มัทรี

There are many online English dictionaries with "speakers".  

"wordweb" (English-English dictionary for Windows) is "free" to download for use "offline".

Please note that words may sound different in UK, US and other countries.