Hi @carmencita!,
As requested, I have put together an explanation that I hope will help you build sentences while talking.
Let’s start with the basics: A sentence contains a subject, verb and an object. The subject in a sentence is generally the person or thing carrying out an action. The verb is a word describing an action. The object is involved in an action but does not carry it out, the object comes after the verb.
Alright, so let’s put theory into practice! 
The boy climbed a tree.
Subject= boy (Q: Who is doing something?/ Who is the sentences about?)
Verb= climbed (Q: What are they doing? What is the action?)
Object= tree (What/Who is involved in the action?/ What comes after the verb?)
When you build your own sentences while speaking, remember the positioning of the words.
In simple sentences:
subject (who is doing something? Who is the sentences about?);
verb (indicating an action)
object (what/ who is involved in the action?).
Here are some exercises to identify the subjects, verbs and objects in sentences: https://www.englishexercises.org/makeagame/viewgame.asp?id=8841
If you want more information on sentence construction click the link to learn about compound and complex sentences.
If you have any further questions feel free to write them here or let me know in our Live Lesson.
Hope this helps! 