I do not have the manual for our Singer Touch & Sew Deluxe ZigZag model 640 sewing machine, serial # AS056384. It breaks the needle when we use it. Is there a special needle that we need to use or is there a special adjustment that we can make. Thanks in advance.

It's a somewhat convoluted procedure. Check needle is not too low. See page 68 in manual.
https://www.singer.com/sites/default/files/product_manual/6dc4cd61b0c61a5fc13e60785f01d29199328059_0.pdf

You really do not need a straight stitch needle plate because you can sew straight or zigzag stitches with the plate that came with your machine. Biggest advantage is that you do not have to change the plate to sew. Many people forgot to change it and broke needles when sewing zigzag.

Search the internet for the

make + model + "manual"

Service manuals are sometimes difficult to find as they were frequently intended only for repair personnel.

You might try some Singer sewing machine groups, ie Yahoo. Group members are usually very helpful.

Check for diagrams on the selector wheel and then adjust your stitch length. Do not set to tight as your thread will break

There is an arm in the head that does the zig zag movement and attaches to the needle bar with a pin. If that isn't connected zig zag won't work at all.

Not much information to go on. What do you mean it won't sew a straight stitch. It just won't sew a straight line? It won't form a stitch? What?

here are 2 links for a manual, that should have the information
inside, one is a downloadable pdf copy the other is a hardcopy

http://www.sewingpartsonline.com/instruction-manual-singer-touch-and-sew-630-and-640-download.aspx

http://www.sewingpartsonline.com/instruction-manual-singer-630-touch-and-sew.aspx

try changing the needle--flat side goes to the back of machine
rethread the machine top & bobbin--make sure to use every thread guide
make sure bobbin is wound correctly & inserted correctly in the bobbin case
clean under the needle plate & in the bobbin area for lint or loose threads
if you don't have a manual try singer
Go to the singer website
Click on customer support--there will be a drop down menu
Choose instruction manual
In the box type the model number(example 758)
Click search

Instruction Manuals

Instruction Manual Singer 625

Most sewing stores will sell. You can buy online as well, some companies will send you a copy to download or mail out a hard copy. There used to be free downloads of books from singer.com but that has ended with the new management.

Derek