The use of the comma

[Tom]

Hi

Could you please tell me if you approve of the use of the (red) commas in the following sentences? I have learned that both are the cases of comma splice--and therefore wrong. However, these sentences have been taken from the Advanced Learners' Dictionary.

[Yankee]

Hi Tom

You ought to know by now that there are precious few hard and fast rules in English when it comes to punctuation.

Personally, I'd probably just omit the comma from your first sentence, and I might insert the word 'that' in its place.

You second sentence is what I would call a comma splice. However, I have to tell you that I really don't see semi-colons used very often, so I'd warn against "over-usage" of them. Instead of using a semi-colon, I'd suggest fixing the comma splice by rewording the sentence this way, for example:
- She is on a short fuse today, so don't irritate her.
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Amy
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ESL teacher, translator, and a native speaker of American English