Hi Eric, thank you for your question. In mandarin usually some auxiliary words, such as “le”, “zhe”, “de”; some modal words such as “ma”, “ne”, “ba”; some nouns with “zi” , like “zhuo zi(table)”, “er zi(son)” ; and some words that are two same characters together, like “ma ma(Mum)”, “jie jie(elder sister)” are toneless. But you also will find some other verbs and nouns are toneless, there’s no rule for them. So you just remember one when you meet it.

No worries, toneless words won’t give you so much trouble when you are speaking mandarin.

you are great, Miss. Tones!