One of the main pronunciation problems I find in Japanese is the tendency to use the schwa sound /ə/ for unstressed vowels, which is common in English pronunciation, but which isn't used in Japanese. So in those examples you cite, for ha-ta-ra-ku, the "a" sound is the same in each syllable, a very definitely pronounced, consistent open-mouthed "a" sound in every syllable. In English, the "ta" syllable would be unstressed, so would be pronounced /tə/, but this is not done in Japanese.
The "u" sound is also quite specific, similar (but different) to a shortened version of the English "oo". "i" is another one that is different in Japanese, with the lips stretched wide, almost like saying a shortened version of the letter E in English.
(Disclaimer: these are just my observations as a student of Japanese. They aren't intended to be a 100% accurate, scientific description of phonology or phonetics. The linguistics experts on this board will no doubt find my terminology and descriptions amateurish. Many apologies in advance for that)