One could write a dissertation on foreign language accent. But here's a little observation I made after my recent reading on phonology. Some Chinese have a hard time to pronounce [ʌ] as in 'but' correctly, substituting [a] as in Chinese "阿" for it. But those living in an English-speaking country long enough can easily make a distinction not only in listening, but in pronouncing it as well. Now comes the more difficult one, the difference between [a] ("阿") and [ɑ] (as in "palm"). I knew the difference and subconsciously made the distinction in pronouncing "阿" and "palm" in its own context and language. But I had not realized the International Phonetic Alphabet actually used two different symbols to represent them until recently I did some casual reading of Wang Li's Chinese Phonology (汉语音韵) and Bernhard Karlgren's book on the same subject. So what's the difference between these two vowels? A good explanation is in the vowel chart of the IPA. For native Chinese, all [ɑ] needs is to move the tongue slightly toward the back from where it is needed to pronounce the Chinese [a] ("阿").
Chinese accent, or foreign language accent in general, in speaking English, is actually easier to overcome when English has a syllable [note] completely non-existing in Chinese (or that foreign language). When there's a syllable that sounds like one in Chinese but does not exactly match it, the native Chinese student learning English will conveniently substitute the Chinese syllable for the English counterpart without being corrected. Short of an incentive to make this correction in his future career or life, the substitution becomes permanent or fossilized.
[note] It would be better to talk about the more "atomic" element, phoneme. But that may be slightly too technical to people that stumble across this blog.