Bajo is an adjective that means short or a preposition meaning under, so you need a noun after, like El perro está bajo la mesa (the dog is under the table). De is a preposition meaning origin or possession. Yo vengo de España ( I come from Spain), el abrigo de Laura (Laura's coat). Del is the combination of the preposition de + the article el, only for masculine nouns. El color del arcoiris (the colour of the rainbow). Hope it helps :)

Del = De + el. "Del" is a contraction of de + el. So the difference is that in the word "del" there are a preposition (de) and an article (el) together. "De" is only the preposition. Between "bajo" and "abajo". Normally "bajo" refers to something that has not a lot of height and we use "abajo" to talk about something that is placed in an inferior place. Sometimes we can use "bajo" with the same meaning than "abajo" (but not "abajo" with the meaning of "bajo") For example: "Este edificio es bajo" (The building is not high) "Mira abajo de la mesa" (Look under the table) / "Mira bajo de la mesa" (Look under the table).

Hola Helen! I will also like to add that we change "de el" for "del" when we are referring to something improper, common, etc. And we use "de el" when we talk about a proper name or a pronoun. For example: 1) No me gustan las noticias del periódico. 2) No me gustan las noticias de "El Nacional" ("El Nacional" es un periódico venezolano). 3) Estoy cerca de él. (Él as a pronoun). Hope this helps! :)