Present tense in the narrative past tense

I've read a number of questions here on SE English as well as from Google searches, however I'm still unclear on something.

Consider the following:

Alice walked carefully along the uneven lawn, making sure to keep her ankle straight.

Bob felt as if he was forgetting something as the bus stopped at the curb.

I'm confident these are both correctly conjugated, but as a native English speaker I also know I have a lifetime of repeating the same mistakes over and over again.

The question is why is it that these two are both correct despite one mixing tenses, and what "test" can be used to determine such things?

Some things I read before asking this:

I've read more, but I'd rather not fish through my history just to show that I'm not just asking due to being lazy.

Answer

In your example

Alice walked carefully along the uneven lawn, making sure to keep her ankle straight.

there is only one verb marked for tense, i.e. establishing when something occurred relative to the time of utterance, and that is the finite verb agreeing with the subject: walked.

The participle making is present in form, but does not itself have tense. A present participle can be used with a finite verb in any tense, indicating that the action/state described by the participle takes place at the same time.

Next year I’ll visit all the major museums in Rome, making sure I have at least three days for the Vatican. [Future]

Hold the board in place, making sure it's perfectly flat. [Present]

A perfect participle indicates a prior action/state relative to the finite verb:

Having first made sure his shirts were properly ironed and folded, John got down to the business of packing.