"I am reading the letters now."
Translation:Yo leo las cartas ahora.
Neither "Estar" nor "Ser" is needed for this sentence.
Even though the English sentence uses the Present Progressive Tense, unless the action is happening right at that instant, the Spanish will use the Simple Present Tense.
English uses the Present Progressive all the time--Spanish rarely uses its Present Progressive Tense.
When you do use any Spanish Progressive Tense (past, present, future), you always use some form of "Estar" + the Present Participle (the gerundio ending in -NDO).
"Yo soy (gerund)" is not a standard tense in Spanish, the present continuous tense uses the verb "estar" instead.
It would be valid in some phoetic contexts, though. For example: "Yo no estoy viviendo, yo soy viviendo" (I am not living, I AM living), which means that the speaker thinks that living is a fundamental aspect of their existence.
"Yo leo..." can be translated to either "I read..." or "I am reading..." or "I do read..."
Although Spanish has its own Present Progressive Tense, it uses that tense sparingly and only when an action is happening at the same time as the sentence is spoken.
We use the English Present Progressive Tense all the time for all sorts of statement--recent past, near-future, planning, etc. In fact we use that tense quite a bit more than the Simple Present Tense.
SO--very often translating the Spanish Simple Present Tense requires the English Progressive to sound normal.
By the way, the Spanish Present Progressive is constructed using the present tense conjugations of "Estar" + a Spanish Gerundio (the equivalent of an English Present Participle ending in -NDO).
"Estoy leyendo" might work here.
The Spanish simple present ("leo") can be (and commonly is) used to translate either English simple present ("I read") or present progressive ("I am reading").
If you want to use the present progressive in Spanish, you are correct to use a form of "estar" but it is then followed by the present participle ("leyendo")
Duo trickle-feeds this through the lessons, but if you want to read up a bit, here are some resources:
"Ahora" says you are reading "now".
But in Spanish, you don't HAVE to use "Estoy leyendo" to say "I'm reading".
"Leo" can be "I'm reading" as well as "I read." and "I do read."
"leo" is the conjugated "I" form. Like if you say "I speak" you say "hablo" "leer" is I believe called the infinitive "to read". So, if you're telling someone, "I want to read", for instance, "Quiero" would cover "I want" and "to read" would be the base form, the infinitive form "to read".
The Spanish Simple Present can be used to translate the English Progressive ("am reading"), so "(yo) leo" is the most common form.
The Spanish Progressive is formed by a conjugation of "estar" (not "ser") plus the present participle so it would be "(yo) estoy leyendo". This form is used much less frequently in Spanish.