In a Country like the Philippines where the English is their 2nd language relying on English to communicate is fine in Thailand you can get by as there are English speakers around but there will be times when speaking Thai is necessary or at least a big help.
For tourists the basics is enough,and it is always polite to learn the basics of the language if you are to be visiting the country.You will encounter many English speaking Thais who will be willing to help you with the basics of the thai language.
Thai Language is tonal but the locals expect you to get it wrong as most farang (Thai for foreigner) find this a new experience in languge,do speak the word the best you can,the locals will have a pretty good idea at what you mean and will laugh it off if you get it wrong and correct you with a example.
Sawat-dee , is for hello and goodbye
Sabai dee mai , is How are you
(reply) Sabai dee , is I am fine
khorb khun , means "Thank you".
mai ben rai , means "it's ok".
khor thot , means "sorry". to interrupt or get past someone
If you want to say no,market stalls included.
I don't want any, mai ow
I don't have any money, mai mee stung
Where are you going? , bai nai
Going out for pleasure, bai tee ow
If a taxi persists say,I want to walk, ja dern bai
You can just speak it as you read it if you want to go a step further you can buy a beginners book with CD thats covers tonals.
For men when you finish speaking a sentance say Khrup if you listen to a Thai man it sounds like cup.
For woman say Ka,these are polite endings to your statment.
One must to learn is the numbers.
Ten : sip
Eleven :sip et
Twelve :sip song
Thirteen :sip sam
twenty: yee sip
thirty: sam sip
100: nueng roi
200: song roi
1000: nueng pun
2000: song pun
Have a quick study before you travel,try it out when you get to Thailand your Asian girlfriend will be happy to teach you as it will help her with her English language skills.