What do you find the most difficult when you started learning Thai from scratch? In my opinion, tonal difference should become a problem for many native English speakers. And if someone continues on, a topic called "คำเป็นคำตาย" will pose a difficulty learning and understanding complicated Thai. Even native Thais are having problem understanding its principle. The issue is so and still great that it makes fun of any people who write "นะค่ะ" (This phrase is readable but there is no meaning. Most of the time, it is misunderstood with a phrase "นะคะ") or even "นี่" vs "นี้." I used to swap the latter 2 words.