I'm always uncertain with the usage of the conjunction and in a situation like these: Should I say When they are too depressed and feeling a pervasive sense of suffering... or When they are too depressed and are feeling a pervasive sense of suffering... or When they are too depre...

Should I use the second "will" in constructions like this one: "it will definitely help you and will make the text more readable" And should I write "to" before every infinitive in enumeration, or only before the first one, e.g. "it helps to develop and test" or "it helps to develop and to test"...

Which is correct (or better) English: "Choose lace in neutral pastel tones or also in grey or black" "Choose lace in neutral pastel tones, in grey or in black" "Choose lace in neutral pastel tones, grey or black"

What of the following is right? "We need to find out..." "...how to lower the costs or how to produce more." "...how to lower the costs or to produce more." "...how to lower the costs or produce more."

Working in the field helps us to learn how to apply theories to solve real-world problems, to apply […], and to […]. Are the "to" after each comma necessary?

What is the origin of the word Idaho? Does it derive from originary Indian language?