One of the examples in the lesson also says "Él esta trayendo a un amigo." But I don't see anywhere where traer normally works with "a" and when I plug "He is bringing a friend" into the translator, it doesn't include the "a". Is there something going on here? Also, now that I think about it, unless the friend is literally with him at that moment on the way to wherever, wouldn't this just be "el presente": "Él trae un amigo"?
In this case it depends. The a is considered technically more correct
When your direct object is a person/animate object (or sometimes a personified object), you also add a - it denotes personhood, so it's commonly called "personal a"
Some examples:
dar de comer al perro = to feed the dog [lit. "to give of food/feeding to the dog] No debes juzgar a los demás. = You shouldn't judge others. Siempre ayudan a la gente. = They always help people. Siempre rescatan a la gente. = They always rescue people. Siempre salvan a la gente. = They always save people. La Biblia dice que debemos perdonar a nuestros enemigos. = The Bible says we should forgive our enemies. Conocimos a mucha gente. = We met a lot of people. Cuido a los niños. = I take care of children. Cuido a los animales. = I take care of animals. Cuido a los ancianos. = I take care of the elderly. Cuido a los demás. = I take care of others. Cuido a mi pareja. = I take care of my partner. Cuido a mi marido. = I take care of my husband. Cuido a mi mujer. = I take care of my wife. Cuido a mis abuelos. = I take care of my grandparents. Cuido a mis hijos/hijas. = I take care of my children.
...And so on. That a is that personal a
There are times when a gets used with certain verbs, but that's a separate thing - though you will see things like llegar a conocer a otros "to come to know others", where llegar a is "to come to + infinitive", while conocer "to know/meet" is a verb that takes a with animate objects
The personal a is considered the most correct and it is preferable that you use it, but just know that if you do miss it, it's not like it changes things so much that people don't understand you. In anything official like writing or reading, they will use the personal a. So I'm not saying it's not important, but just know you aren't being misunderstood because you miss a particle here
[I will say that there are a handful of verbs where changing the preposition will change the meaning - as an example vengar a alguien is "to avenge someone"; while vengarse de alguien is "to take revenge on someone"... so if you're trying to do something for someone who was wronged, you see the a... but if you're trying to punish the person responsible it's reflexive and uses de]
It can be a bit confusing for non-native speakers to come to understand what verbs would use it and which ones maybe don't; and I still struggle with it every so often
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As far as trae vs. está trayendo it's a matter of preference/style here
But first some basic things
First, you're right trae can be used in place of está trayendo
Second, trayendo is a form of traer called gerundio which is like "gerund/progressive", or the -ing form of a verb... most end in -ando or -iendo, traer is a bit weirder in that it ends in -yendo
...
Now this goes back to the first point - present tense has three general functions: regular declarative statements, present continuous, and short-term future
Meaning that trae could be interpreted as "brings", "is bringing", or "plans to bring / will bring (shortly)"
The gerundio form is ALWAYS continuous and implies movement or continuous motion, usually used with estar or a verb of motion
Simply put trae can be "brings" OR "is bringing"
But está trayendo is ONLY "is bringing"
...Whichever one you choose is up to you, to me the use of gerundio feels more like immediate motion like "currently"
[Also gerundio can be used in other tenses - estaba trayendo "was bringing", estará trayendo "will be bringing" etc. which could be used instead of traía "was bringing" or traerá "will bring"]
