commas are difficult. In the above sentences I don't think they are required. Others may differ in their opinions and I'm sure a fight will ensue.
Use commas where you would take a
small breath or pause when speaking is a rule I was taught at school. This is probably not helpful to non english speakers.
Common introductory words that should be followed by a comma include yes, however, well.
Well, perhaps he meant no harm.
Yes, the package should arrive tomorrow morning.
However, you may not be satisfied with the results.
More comma usage stuff
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