Here are something I want to point out, 1、“今天这么热”=“今天那么热” here,"这么“、”那么“ both mean “very" or "so....that", what I wan to make you notice is when you say "今天这么热” it implies that another sentence should be followed,like"今天这么热，你为什么还要出去？“ 2、“这么”、那么“ have a second meaning,which equals 这样、那样. they are equivalent of pronouns. 这样=this case , 那样=that case ,so they are used to be compared. For instance,we normally say "你觉得是这样好还是那样好？" 我看就应该这么做=我看就应该这样做 你别那样想=你别那么想 Just my own opinion,hope it works~

这么 - like this, in this way, so 那么 - like that, in that way 为什么中文这么难？ - Why is Chinese SO difficult? (or Why is Chinese difficult LIKE THIS?)

1.表示程度，both mean very. 今天这么热。今天那么热。 这么，那么can exchange in this sentence. 2.表示方式，修饰动词。They show manner ，modifying verbs. 我看就该这么/那么做。你别这么/那么想。 I hope can help you.

Can you give more details?such as some sentences?because you will understand well in context~

这么 = such, like this, 那么 = like that