"speak softly" is correct and usable in written English. You can use it when you want to tell someone to speak in a quiet, gentle voice. For example, "Please speak softly, because my baby is sleeping.".
If so, speak softly.
He did not speak softly to sin.
Every time, it was "Speak softly".
Plenty of brands, even ubiquitous ones, speak softly.
Rabbi Praver began to speak softly, he recalled.
"Speak softly, and carry a big stick," said Theodore Roosevelt.
Who said, Speak softly but carry a big stick?
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Head Translator and Proofreader @ organictranslations.eu