What is really stated is that the advance guard will be with the main body after 7 a. m.—an unintended statement. Make your grammar accord absolutely with your meaning. Add the with which belongs before the advance, and notice how the sense is brought out. Again,
They order us to go to Brownsville and do impossible things.
In this sentence do they order us to do impossible things, or do they themselves do impossible things?
Add the sign of the infinitive where it belongs before do. And to do impossible things reads unmistakably.
3. Do not use a participial phrase without first inspecting it to see that it holds but one idea.
Having changed our position, the enemy was confused.
What this sentence really says is that the enemy, when he had changed our position, was confused. This meaning is evidently not intended from the very nature of the statement.
It is clearer and more accurate to use a finite form of the verb instead of the participle; as,—
Because we changed our position we confused the enemy.
Or if you can condense with accuracy,