Both characters have exactly the same shape if one was rotated by 180 degrees. Usually the three lines are straight, but the longest line may sometimes be slightly curved. The character is the same in all Batak scripts.
The characters ᯤ [i] and ᯥ [u] can only be used at the beginning of a word or at the beginning of a syllable. The word sai (the /ai/ is NOT a diphthong), which consists of the syllables sa and i, should be written ᯘᯤ. The word haudanan ‘get caught in the rain’ consists of the prefix ha-, the root u-dan, and the suffix -an and must hence be written ᯂᯥᯑᯉᯉ᯲. In igung , which is structured i-gung, only [i] can be written with the character /i/ while the sound [u] must be written using a diacritic (see Lesson 8): ᯤᯞᯮᯰ
If you are not sure what constitutes a syllable and what not, just follow the simple rule: if [i] and [u] are at the beginning of a word or if they follow a vowel, then you must use ᯤ and ᯥ. In the word mardai there is a vowel [a] before the vowel [i]. Therefore mardai is written ᯔᯒ᯲ᯑᯤ. Also the word mauli shall be written ᯔᯥᯞᯪ. Only the [u] may be written with aksara /u/ whereas for [i] we have to use the diacritic.
But what shall we do in a word such as parulian? The syllable structure is par-u-li-an because par- is a prefix. Therefore we must write it ᯇᯒ᯲ᯥᯞᯪᯀᯉ᯲. However, in Batak manuscripts we also often encounter the spelling variants ᯇᯒᯮᯞᯪᯀᯉ᯲ or ᯇᯒᯮᯞᯪᯛᯉ᯲.