We previously noted the use of the word чтобы in sentences like:

In the Russian version of that sentence the чтобы clause contains a perfective verb to express a one-time action with result to the point of discourse:

That's all well and good, but what happens if the opposite is the case? In other words, we want to say:

In this case the verb in the чтобы clause must be expressed in the imperfective:

Alternatively, one could put the не right in front of писала instead of in front of хочу:

For beginners the rule of thumb is this: a negated verb will be expressed in the imperfective. In these sentences the scope of negation seems to include not only хочу but also the complement of хочу, i.e. the чтобы clause. Here are some other examples. Notice all the sentences in blue have a perfective чтобы clause. All the sentences in yellow are negative and thus must have an imperfective чтобы clause: