Hi Lucy,
Lisa’s explanations are all great although I would alter the explanation for “convert/transform/turn into”
First, I wouldn’t say that “transform” necessarily has a “positive” connotation, in that anything can “transform” into anything.
I would argue that the difference between “convert” and “transform” is that “transform into” and “turn into” are usually limited to physical properties/appearance/shape etc. (“TransFORM” — change FORM), while “convert into” is used to describe either intangible changes.
Therefore, we convert from one religion to another because it is not a physical change, but an intangible one.
We can “transform” a warehouse into apartments, but we can also “convert” the warehouse or “turn” the warehouse into apartments.
We can use “transform into” and “turn into” because the building is being physically changed, and we can use “convert” because we are also changing something intangible—the way the building is used, or its purpose. The building has been “repurposed.”
When speaking about religion, beliefs, ideologies, political support, etc. we would use only “convert.”
If a man can turn or transform into a wolf, he is a werewolf. —here we do not use convert!
We can “convert” documents on computers—convert text file to PDF. Here no physical change takes place, and so we use “convert.” (This is not a tangible change: if we print out the text document or the PDF, it will become physical, but both types of file will be identical when printed.
There may be some exceptions (as always with languages) but I would argue that if something visibly changes physical form and appearance, “transform” and “turn” are used exclusively—“convert” is never used in these cases unless the transformation also changes the use or purpose of an object which is not a visible change—in which the conversion refers to this other change in purpose/use.