Take a coin and flip it. If the result is heads, pick Fe. If the result is tails, pick Fi.
The descriptions Maritsa has supplied you with could remotely work in an MBTI context, but they don't apply here. Put simply, Fi is the IM (function) of "relationships," in the sense of recognizing psychological distance and focusing on the status of a relationship with someone. (in a simplified manner, recognizing friend v. foe) By contrast, Fe is the function of "emotion" in the sense of recognizing the moods of others and stimulating the emotions of others.
Of course, that is a description of using either as a
strong function. And Ethical ("F") type is strong in both Fi and Fe, while a Logical (T) type is
weak in both, so the above description should help little.
The issue here is function
value. A "T" type, someone with weak Ethics (note the capital letter; "Ethics" here merely refers to the function) values behavior in others that establishes their valued form of ethics in the environment.
I encourage you to read the
wikisocion descriptions for the various functions. Function value comes in pairs. (IE: Te/Fi are always valued together, as are Ne/Si, Se/Ni, and Fe/Ti) For a contrast of behavior between Te/Fi and Fe/Ti valuing behavior, I recommend you read this page on wikisocion:
Judicious (Fe/Ti) and Decisive (Te/Fi)
Maritsa's egregious misrepresentations of Fe and Fi are simply too "off the mark" to point out every instance where they stray from the actual functions. It's simpler to point out a more correct understanding.
As Aiss pointed out, any "newbie" should be wary of Maritsa. She practices her own convoluted idealized version of Socionics, widely rejected by virtually everyone on this forum. One may read what she writes, but this must be held in the context that what she says carries virtually no weight regarding Socionics and should not be mixed with an actual understanding of the theory.