I speak Mandarin. I don't understand Cantonese.

It depends. I don't think people who have never be exposed to Cantonese environment will understand it. Many traditional Chinese characters are adopted in Cantonese, including the rare words. The pronunciation in Cantonese is quiet different from Mandarin. I speak both Mandarin and Cantonese. Even for people like me who have been living in Guangdong for a long time, it's a little hard to understand some Cantonese words. (Maybe because I don't often use Cantonese.) Cantonese is a dialect. Same thing is expressed in different ways. Let me give you some simple examples. 1)When we're in triumph or feel cheerful and confident, we say 得意洋洋 in Mandarin but 得戚 in Cantonese. 2)当然 in Mandarin and 梗系啦 in Cantonese, which means of course or definitely. 3)回家 in Mandarin, 翻屋企 in Cantonese, go home in English. Now we see, even if we can read Cantonese, we are not able to understand its meaning, let alone its pronunciation. Since many Chinese from other parts of China have worked or worked with Cantonese speakers, they more or less know some Cantonese. Once we get involved in Cantonese, to understand some simple Cantonese is not that difficult.

听不懂，写可以猜出大部分

会说白话哦，但是说的不好

Without training or learning, the answer is absolutely NO.