Just to add to what the others have said, quidam, quaedam, quodam is an indefinite pronoun used by the speaker or writer when he has some definite instances in mind of whatever he's talking about, but does not wish to specify them to the listener or reader, either out of politeness or because he doesn't wish to reveal anything or feels it unimportant. It is often translated "certain" or "a certain" (singular), though that sometimes sounds over-translated in English. You may instead translate it with the word "some", but when doing so you have to keep in mind that the meaning is more particular in Latin than in English. You would not use quidam when "some" means "some or other", i.e. when the speaker/writer is himself not referring to anyone/thing as definite, as in "some men are evil" = "there are men who are evil", as opposed to "certain men (but I won't mention their names) are evil". Here you would instead use aliquis, quispiam (somewhat rare), or for special emphasis nescio quis.
It should also be carefully distinguished from the use of "some" = "a few", as in "I have some money". In such case you would instead use a double negative form in Latin, as non nulli "some/a handful of men" or non nihil pecuniae "some money [literally something of money]".
The force of quidam is sometimes so weak that it may be accurately translated with nothing more than the English indefinite article (in the singular), or not at all (in the plural).
Ex mala malo
bono malo uesci
quam ex bona malo
malo malo malo.