I will use your sentences to explain you the grammar. Otherwise I would use a different structure to express these sentences, but your main concern for now is the grammar, as far as I understood. 'Denn' and 'weil' may have a common level of meaning in the German sentence, however, they are used completely different. Both words are conjunctions, but 'denn' can introduce only a main clause and 'weil' only a subordinate clause. This is visible in the position of the finite verb in the sentence. Example: Ich falle, denn die Schwerkraft ist immer noch in Kraft. --> Ich falle, weil die Schwerkraft immer noch in Kraft ist. Ich falle, denn ich bin gesprungen. --> Ich falle, weil ich gesprungen bin. denn belongs to the subordinating conjunctions (denn + main clause) as the following as well: aber, allein, doch, nur, oder, sondern, sowohl. weil belongs to the coordinating conjunctions (weil + subordinate clause) as the following as well: als, bis, da, damit, dass, ehe, nun, ob, ungeachtet, wenn, wie, zumal, obwohl