Whilst the tonal nature of Thai presents considerable challenges to speakers of atonal languages, its grammatical structure is considered by most to be fairly straightforward. The principal difference is that verb forms do not vary to show first or third person forms, or to show differences between, for example the past or future. First and third person forms are generally understood by their context or by specifying 'I' or 'He' and markers are added to indicate tense. For example, 'ja' for the future or 'ma' for the past. Thus, 'Tomorrow he's going to see a movie' in Thai is 'Prung ni Khao ja bai duu nang' or 'Tomorrow he [future marker] go see movie'. Thai also has no articles as illustrated by this sentence. For Thai speakers of English this makes grammar, particularly aspect, a very difficult part of the language to master and there is frequently an over reliance on the present simple tense used in conjunction with a time phrase.