I have a last doubt.

Thank you
I taught to my students (or my students?) how to speak?
I'll write to you soon.
I can't understand why in English the indirect object is not always preceded by "to". Can you give me examples? Thank you.
He gave his life to his fatherland. Or He fell for his homeland.

2 answers

I taught my students how to speak.

By definition, an indirect object is not preceded by "to." Indirect objects always come between the verb and the direct object, and often follow these verbs: give, bring, teach. This site has more examples.

http://www.slu.edu/colleges/AS/languages/classical/latin/tchmat/grammar/g-indob.html
Indirect objects:

I gave him a cup of coffee.
I taught my students.
The waitress brought us or salads.
If you have studied Latin, you'll find the same thing. Indirect objects are in the dative case and have no preposition.

Latin prepositions are used either with the accusative or the ablative case, depending on the meaning. (Usually any preposition implying movement toward something is followed by a noun or pronoun in the accusative case.)