Maria, I think this lesson teaches the use of the verb estar, so you're getting a lot of estar sentences here. That includes greeting phrases (like "¿Cómo estás?"), but is not limited to them.
It's sometimes hard to find a title that covers the things that are taught in the lesson.
Do you mean ésta/éstas or está/estás. The accent placement is very important here, since those are two entirely different word groups.
Ésta and éstas are demonstrative pronouns, meaning "this" and "these", respectively. They are used to refer to one or more feminine objects, as in "Ésta es mi bolsa" - "This is my bag." The corresponding masculine forms are éste and éstos. It is now recommended that all these forms are written without the accents: esta, estas, este, estos.
Está and estás are conjugations of the verb estar, which translates as "to be". Está is the él/ella and usted conjugation (meaning "he/she is" and "you are", with the "you" addressing a person in a formal manner). Estás is the tú conjugation, for addressing a single person informally: "you are".
In this sentence you have a "he" as the subject, so the form está is used.
El without an accent is the singular masculine article, "the".
- el hombre - the man
- el tren - the train
- el coche - the car
- el árbol - the tree
Él with the accent it the personal pronoun "he".
- Él quiere ir al parque. - He wants to go to the park.
- Él no habla conmigo. - He doesn't speak with me.
Rishav, you seem to be mixing up verbs and adjectives. Verbs conjugate, which means that their ending changes depending on who carries out the action:
- yo hablo - I speak
- tú hablas - you speak
- él/ella habla - he/she speaks
But cansado is an adjective. Adjectives care about something different, specifically the gender and number of the thing it describes. While verbs describe actions, adjectives describe objects.
- Él está cansado. - He is tired. (singular masculine form)
- Ella está cansada. - She is tired. (singular feminine form)
- Ellos están cansados. - They are tired. (plural masculine form)
- Ellas están cansadas. - They are tired. (plural feminine form)
Yo can take either cansado or cansada, depending on whether the speaker is male or female.