PCR requires two primers. What outcome is expected if one of the two primers is mistakenly omitted from the?

You will produce much less DNA, and the fragments that you produce will not be of a specific size. The two primers define the size of the amplified region.

Polymerase cannot attach to DNA without a primer, so PCR is useless without them.

Primers are small fragments of DNA which are added in the PCR mix in order to bind to the single stranded DNA at various locations and carry on synthesizing the double strand till the end.

Their sequence determines where they bind on to the DNA how many new fragments of DNA are formed at the end of PCR.

PCR needs to primers, each complementary to a different strand, because the primers define the region that will be amplified. In the first cycle, the double stranded DNA is heated, so that the hydrogen bonds holding the 2 strands together break, and the 2 strands fall apart. When the temperature goes down, complementary sequences will start to bind to each other. Because there are many more primers than template DNA, the chance that a primer will bind to its complementary sequence is much higher than that the 2 original full length template strands will find each other.

One primer is designed to bind on one strand, the other primer will bind the other strand. The enzyme DNA polymerase will extend these primers, and use the template as a - well - template. DNA synthesis always has a direction, and if the primers are designed well, both newly synthesized strands will run towards each other, and then pass each other.

In the second cycle, primers will bind to these newly synthesized strands, and the first real PCR products are formed.

The link below has a (slow) animation of the whole process. That is probably more helpful then me trying to describe it in words.