La semana empieza el lunes/the week starts in monday

I don't know the word for "when" in Espanol" so -

?When dice "el" en Espanol para "in" en Engles y when dice "en" para "in" en Engles?

In English:  When do you say "el" in Spanish for "in" in English and when do you say "en" in Spanish for "in" in English?  Or, what is the difference between "el" and "en"? 

En Engles las dias es en upper case, or capital, letters (i.e. Monday, Tuesday, Wednesday, Thursday...).   ?Esta en Espanol las dias en lower case letters (i.e. lunes)?

Days=dias
Semana=week
Meses/months

I don't know the words in Spanish for "if" or "then":

?If "dias" en Espanol esta "days" in Engles then esta "dia" en Espanol "day" en Engles?

?If "semana" en Espanol esta "week" en Engles then esta "semanas" en Espanol "weeks" en Engles?

?If "meses" en Espanol esta "months" en Engles then esta "mese" en Espanol "month" in Engles?

Again, this is not one that I can even attempt to ask in Spanish - When do you use the word "es" and when do you use the word "esta"?