th difference between hace -v- desde hace

Question

A valuable lesson! But can you please explain the difference in the use of "hace" and "desde hace"? It seems that "hace" means "ago":
Lo compré hace tres semanas. - I bought it three weeks ago. 
And that "desde hace" means "for": Lo tengo desde hace tres semanas. - I’ve had it for three weeks.
However I keep coming across examples where either "desde hace" or "hace" can mean both "for" or "ago". Does it depend on context as to which one means "for" or "ago" or are they interchangeable?
It seems that translation is dependent upon interpretation:
"que no saldrá del cajón y le saldrán telerañas" translates as "which won't leave the drawer and it leaves with cobwebs" BUT is interpreted to mean "which will stay in the drawer gathering cobwebs". 
I note that in Mexico, official documents have to be translated "literally" but in the case above, that really wouldn't make much sense! How far can one go in interpreting versus translating? 
xx

Inma Sánchez · Accepted Answer

Hola Gary
It's not often possible to translate idiomatic translations literally so, in these cases, we translate in the most natural way to express the same thing. In this part of the text saying "...que no saldrá del cajón y le saldrán telerañas" we're translating it as "...that will stay in the drawer gathering cobwebs..." because if we used a word by word translation [...that won't come out of the drawer and cobwebs will come out to it.] wouldn't make any sense in English. The expression "salir telarañas a algo/a alquien" is used when talking about something that is hardly used, neglected, or simple inactive. It's used in a lighthearted way. This is what we mean in the text. Maybe if we said in English "gathering dust" it'd be a bit clearer, not sure.
As for desde and desde hace, you may have found some more flexible translations where they alternate for and ago, but I can't really think of any example where hace and desde hace can be interchangeable. Desde is followed by a specific time: desde 2009, desde los años 80, desde la época de los romanos...;desde hace is followed by a "lengh of time" instead: desde hace 2 años, desde hace un siglo, desde hace diez minutos...
I hope this clarified it.
Saludos
Inma