I studied French in school for seven years, forgot most of it in forty years, and relearn it now from an App. One thing I struggle with are pronunciations / liaisons in specific cases, for example:  
_Elle es**t**  en form_  
or  
_Je sui**s** allée_  
(I marked the letter in question as bold)

In the app, they seem to pronounce it sometimes this way and sometimes the other way - so my question is:

**Are there clear rules when to liasion (pronuncing that letter) and when not?**  
How flexible is this in reality? Do I stick out like a sore thumb if I do it the wrong way around?

PS: I am aware of different pronunciations and spellings depending of the subject / object being male or female. However, specifically in the “Je suis allée”, in the App the male speaker pronounces the ‘s’, and the female speaker doesn’t, which seems to be the wrong way around (or random).