I am learning the duolingo English course. In some sentences, they use In front of, but the others use In the front of. Would you please let me know what is the difference between those two? Thanks
In front of is a common phrase which functions as a compound preposition: in modern English it has almost completely replaced before in a spatial sense. If you try to analyse it, you need to interpret "the front of" as referring to a space outside the thing, which is a relatively uncommon meaning for it.
In the front of is not an established phrase in English, and so would normally be interpreted literally: "inside the front part of something". It is not common, because "the front of" something is usually a surface, and we don't often talk about things being in surfaces. An example where it might occur is I've got some water in the front of my watch, where we could understand "the front of my watch" meaning the space between the glass and the watch face.
In front of means the item is outside something, but in the front of means it is inside.
For example, a building can have a front and a back and someone standing before it would be in front of the building. Someone standing inside the building but in the front half, would be in the front of the building. Similarly they could be in the back of the building.
Another example would be in a race where a runner could be in front of the pack, which means they are ahead of the main group of runners, or they could be in the front of the pack, which means they are within the main group, but near the front, but probably not right at the front.
Oddly we don't say in back of very often, it's usually behind, but almost always in front of rather than before which is normally used when talking about time.