は and が the basics

Note: This entry will be made in two parts. This is the first half of the entry.

Most of us have had at least one friend or teacher who, in the spirit of being helpful, tried to explain the subtle differences of は and が. Unfortunately, most of the time this only led to us getting more and more confused.

But fear not, for the division is actually quite clear.

Rather, according to my "Prepartory Course for the JLPT:　文法・語彙集" book by Unicom, the baffling division between は and が isn't nearly as arbitrary as many of us have been lead to believe.

A couple of rules:

A. When using は, the most important part of the sentence / thing you wish to convey should be after は. For が, most important part of the sentence / thing you wish to convey should be before the が.

は
あの人は誰ですか？
あの人はスミスさんです。
Here, the name is what A wants to convey. Therefore, スミスさん is logically the most important part of the answer.

が
どの人がスミスさんですか？
あの人がスミスさんです。
Here, the どの人 is that A wants to convey. Therefore, あの人 is the important part of the answer.

B. When using interrogatives,　は is placed before them and が should be placed after them.

何が分からないのですか？
分からないのは何ですか？

C. は affects the entire sentence, essentially, since it is the subject marker. が, however only affects the closest verb.

私はアメリカへ行く時、空港で財布を盗まれた。アメリカに行く：　私
財布を盗まれた：　私

母は私がアメリカへ行く時、空港で財布を盗まれた。
アメリカに行く：　私
財布を盗まれた：　母