One common feature of the German language is that it often has several words with the same translation in English. A case in point is the words "weil", "da", and "denn" all of which translate as "because". However, that is not to say that they are used in exactly the same way- they aren't.
While both "weil" and "da" are subordinating conjunctions that send the conjugated verb to the end in a dependent clause, "denn" is a coordinating conjunction that doesn't affect word order in German. The second difference is that "weil" is the most common word that is used at the start of subordinate clauses in which reasons for certain actions or statements are being provided like the English word "because", "da" and "denn" are stylistic alternatives which are used in texts to mix things up, similar to the English "as", "since" and "for". Let's look at examples to clarify both points.
Ich lerne Deutsch, weil mein Partner aus Deutschland kommt (I'm studying German because my partner is from Germany)
Ich lerne Deutsch, denn mein Partner kommt aus Deutschland (same translation, but notice the difference in word order. Whereas the conjugated verb "kommt" is at the end of the subordinate clause after "weil", it is second in the "denn" clause that links two main clauses).
Er ist nach Berlin umgezogen, weil seine Freundin dort wohnt. Da er die Stadt auch liebt, war es für ihn keine schwierige Entscheidung (He moved to Berlin because his girlfriend lives there. Since he also loves the city, it wasn't a difficult decision for him).
As the above example shows, "da" can be used as a stylistic alternative to "weil" when reiterating the latter would be too repetitive. "Denn" could also have been used in the second sentence, but not at the start because coordinating conjunctions are not used at the beginning of sentences. This is just a convention, though, just like English native speakers don't start sentences with "but". So here is the sentence with "denn": "Es war keine schwierige Entscheidung für ihn, denn er liebt auch die Stadt". Finally, many German students confuse denn with dann sometimes, so read my blog post on the difference between them to avoid confusion.
On our German language blog "Auf Deutsch, bitte!", you will find helpful posts about other topics, such as German cases, prepositions, and German word order.
Very very good Erklärung. Vielen Dank.