PUT THE VERBS IN BRACKETS INTO THE CORRECT TENSE FORM>
I (to write) the exercise before you come .
Before realizing his plan, he (to work) much at it.
After publishing his articles about space travel in 1893, Tsialkovsky (to finish) his first complete work.
They (to spend) all the money by the end of the month.
Our scientists (to study) thoroughly the problem of space travel now.