Why are certain verbs always followed by the dative case?

In a sentence, the object can be either direct or indirect. When the object is acted on directly, it takes the accusative case. Example: 'Sie isst den Apfel' (She is eating the apple). In other sentences, when the action is being performed to, or for the object, the object becomes indirect, and therefore takes the dative case. The clearest example of this in English is when the word 'to' is present in a sentence. Example: 'Er hat dem Kind gesprochen' (He spoke to the child). Certain verbs that are indirect in German do not appear to be indirect in English: two such examples are: 'Ich helfe der älteren Frau, die Straße zu überqueren' (I'm helping the old lady to cross the road). 'Der Lehrer bringt den Studenten Deutsch bei' (The teacher teaches the students German)