SuttaCentral
(1) There are so many words such as bhikkhu, phra, ajaan, bhante, etc… Are there any differences among them?
(2) ‘Than Ajaan’ usually refers to Ṭhānissaro Bhikkhu (Ajaan Geoff), but I heard from somewhere that ‘Than Ajaan’ can also be used for other monks. I feel confused. The word ‘thanissaro’ is not a proper noun but a regular noun?
(3) I see ‘EBT’ a lot in this website. What does it mean?
Best,
Heesoo 
1 ) Bhikkhu means a fully ordained monk. It’s not usually used as a form of address, though. So you can say “I enjoy Bhikkhu X’s talks” or “A bhikkhu has 227 rules”. But it wouldn’t sound right to say “Hello, bhikkhu”.
Bhante can be used as both form of address and a way to refer to a monk. So you can say “Hello Bhante, how are you?” and “I met bhante X today”. But you can’t say “a bhante has 227 rules” because it doesn’t mean a monk in every context.
Ajahn means teacher in Thailand. In the Thai tradition, a monk who’s been in robes for 10+ years can be referred to as such. Than/Tan is an additional honorific to specify that its a monk because ajahn can also mean a regular teacher like a school teacher.
2 ) Than Ajahn can be used for any monk in the Thai tradition with sufficient seniority. It has no special connection to Ajahn Thanissaro, although it applies to him.
I’m guessing the confusion here is that in the context of a specific monastery, monastics are sometimes referred to with honorifics only if it’s clear who they refer to. For example, if Ajahn Thanissaro is the only senior monk in a given location, then people might just call him Than Ajahn, omitting Thanissaro, because its already clear from context that they’re talking about him.
3 ) Early Buddhist Texts. You can read more about what that means here: SuttaCentral
BTW, there are already several threads here on the forum on this topic. Try the search feature to find them.