What is the difference between "el mismo" and "lo mismo'?
How do I know which one to use?
'lo mismo' means 'the same thing' - ex. 'Ella se calló, yo hice lo mismo' - She shut up, and I did the same.
el mismo/la misma - is refered to thing - el mismo día (the same day) - Mi coche es el mismo que el tuyo. - My car is the same as yours.
El mismo can be an adjective or a pronoun.
El mismo or La misma (adjective) + noun Also, Los mismos or Las mismas (adjective) + noun
El mismo amigo (the same male friend) or La misma amiga (the same female friend)
El mismo or La misma (pronoun) Also, Los mismos or Las mismas
No quiero éste, quiero el mismo / No quiero esta, quiero la misma
Lo mismo. There are at least 3 different forms of using it. I will discuss the first one (the same thing). The other two plus a general discussion of mismo can be found here
Siempre me dices lo mismo (the same thing -- lo is a neutral article and it is used when the subject is unknown)
Me da lo mismo (Same thing or same difference, like we say in American English)
Where are the pictures that go with the word of the day that people submit?
It would be easier to explain with context, but I think él mismo would be himself. With el mismo, you might have a noun following like el mismo sábado or la misma cosa... the same Saturday, the same thing. Lo mismo is just the same or the same thing. It isn't specific.
Have a look at this thread.