Tutor profile: Rohn W.

How do you know when to change, or not to change, the stem in a stem-changing verb, such as tener or empezar?

It depends on which syllable the stress falls on in the conjugated form. In regular verbs in the present, the stress falls on the penultimate (next to last) syllable. If the penultimate syllable is in the verb stem ("ten" or "empez") then the stem gains an extra "i" before the "e" in that penultimate syllable. Examples: yo empiezo tu empiezas el/ella empieza nosotros empezamos ustedes/ellos empiezan yo tengo (irregular form) tu tienes el/ella tiene nosotros tenemos ustedes/ellos tienen The short answer would be "you don't change the stem on the nosotros form." But that leaves out the reason why.

What is the universal donor blood group for humans?

The short answer is: Group O. There are 4 ABO blood groups in human beings: A, B, AB, and O. A and B are the names assigned to red cells carrying specific sugars on red cell membranes. The human immune system recognizes the differences between these sugars and will create antibodies against substances not present on one's own red cells. Persons having group A blood will naturally build antibodies (anti-B) in their plasma against group B red cell substance. Conversely, individuals with group B blood will build anti-A. Group O individuals are like "plain vanilla," lacking either A or B substances on their red cells, so they create both anti-A AND anti-B. Persons with group AB blood have BOTH substances present, and do not produce anti-A nor anti-B. Since group O persons have "plain vanilla" red cells without A or B substance, it is perfectly save to transfuse group O red blood cell units to A, B, or AB patients with no compatibility problem, under normal circumstances. At this point, you should not find it surprising that group AB individuals are the universal recipients for red blood cells, safely receiving group AB, A, B, or O red cells for transfusion.

Is it wrong to use the passive voice in English?

While many software-based grammar checkers will question the use of passive voice in English, the fact is that sometimes it's going to be the best way to phrase an idea. It is important to consider if the subject of the phrase or sentence is actually doing something itself as opposed to having something done to it. Here's an active voice example I've seen quite a few times in the news lately: If you don't want to catch Covid, you should vaccinate. It's simply incorrect! "To vaccinate" is to inject or otherwise expose an individual to a vaccine which will hopefully prevent an illness. The person who wants to be protected doesn't vaccinate; he or she is vaccinated. The recipient of the vaccine is passive; thus, passive voice is perfectly acceptable, and in fact, preferred in this situation. If you don't want to catch Covid, you should be vaccinated. This works much better in this instance.