John, 

The addition of ‘t’ is only for euphony. It has no meaning otherwise. 

There is no need for the ‘t’ in “apprend-elle” to have euphony, as it comes directly from liaison ( not elision) of the terminal ‘d’ of apprend (pronounced as ‘t’) with the following ‘elle’ commencing with a vowel. 

The ‘t’ is only needed when the preceding verb form ends in a vowel - see the 2nd ‘green’ section ’ in the lesson that you linked.