I test out when I feel I have mastered a level. So instead of 70 lessons on a skill, I may be ok with 50. And I try to have 4 skills active at the same time so I don't get bored. But repeating the same sentences many times, though boring, helps memorize the structures amd vocabulary.
To answer your original question, yes, there is a reason for this. When vav is in the middle of a word or at the end, it is usually representing a vowel sound. So in many words the vav is doubled to show you when it is being used as a consonant. But at the beginning it is always a consonant so you do not need to double the vav. The exception to the rule from the pronunciation point of view is when the vav is an "and"...this can be pronounced ve or u, but there are precise rules about which it is going to be, and ve is still the most common pronounciation. For now you just have to remember the exceptions. There are some differences between modern Hebrew and Biblical Hebrew, but the adding on of "and" at the beginning of the word has not changed, so the spelling in this case is respecting tradition.
The plural of fish is usually fish, but fishes has a few uses. In biology, for instance, fishes is used to refer to multiple species of fish. For example, if you say you saw four fish when scuba diving, that means you saw four individual fish, but if you say you saw four fishes, we might infer that you saw an undetermined number of fish of four different species.