Baby sweater

[Dianea]

I don’t understand what they want in this pattern.  
I have finished with the back of 33 rows and 75 hdc.  The pattern reads for back sleeve shaping: Row 1 - right side Ch 8 turn work 1 sc in 3rd st from hook,1 sc in next st of ch, 1 hdc in each of 4 sets of ch,(HERE IS WHAT I DONT UNDERSTAND)         
1 hdc in eac st of row below; (DOES THAT MEAN 1hdc  ACROSS 75 sts) then joins a separate piece in top of turning ch of row below, just below last st worked, ch 7 fasten off and break this yarn; work 1 hdc in each of next 4 sts of ch, 1 sc in each of 2 sts, 1 sl st in last st: 89 sts added each side.

 

[Granny Square]

Welcome to the 'ville!

It sounds like your garment is "T" shaped, where the bottom of the T is the back body, and the top cross part is 'cuff to cuff', from one sleeve edge, across the top of the body, to the other sleeve edge.  This is what I think is happening:

So far you have made the bottom body part of the back, across 75 HDC.  The sleeve is going to extend beyond the body, and you are at the point where this is about to happen, so you have to create a foundation chain for the sleeve - that is what the 'chain 8' is for....if you chain 8, turn, SC into the 3rd chain from of the hook you should have 5 chains remaining;  the pattern says to SC in the next chain, and HDC into the next 4--so it all adds up.

Then you work 75 stitches across the row of the body in HDC, as you guessed.  The next step is to extend the sleeve for the other side by attaching the yarn in the top of the row below and making a foundation chain for the sleeve, turning, and working back across the row.  From now on you will be working across 1 row which crosses 1 sleeve, the body, and the other sleeve.

There is another way to do the part where I underlined 'the next step' above, if you are familiar with foundation stitches - you could just make foundation HDC stitches to extend the row you are working on by a matching # of stitches for the other under sleeve.