Hi, Doug. That's an interesting question, I'll try to explain it. "Como a" is an expression that we use when we're not being exact, but approximate, about the hour. For example: -La reunión era a las 12 -> We're sure about it. -La reunión era como a las 12. -> We're being approximate. But it's a colloquial expression. To be more formal we would say "La reunión era sobre las 12" o "aproximadamente a las 12". About the use of "un", it's difficult to explain it. When you say "soy ingeniero" or "toma asiento", we understand the "un" (I'm not going to take two seats, for example, I understand that is a single seat). But I had never thought about why in some expressions we say "un" and in other expressions we don't... I only can assure you that when talking about professions we don't say "un", we always say "soy médico", "soy profesora", "soy cantante", etc. I hope this helps you a bit. Have a nice day.

Hi Doug: Just to comment Esther's answer about the use of "un". At least you say: "Soy un ingeniero como muchos otros" / "I'm an engineer as many others" or "Soy un ingeniero que cambiará el mundo" / "I'm the engineer that will rock the world" then "un" is completely unnecesary. In other words, If you are just going to tell your profession , the "un" should not be used. But, If you are going to use your profession to make a sentence, an exclamation or an statement, then it is mandatory. For "toma asiento", it's like Esther said, since your are supossed to take one seat we don't use "un". But if it's necessary to remark to take just "one" you can say "tome un asiento", "coge un cigarrillo", "toma un lápiz". Sometimes "un" is necessary to be formal, Maybe it will happen that you think the other person will take advantage of the situation. Also sometimes we use "un" because otherwise it will not sound good. For instance, "toma un vaso" (de cerveza) / "Have a drink". You cannot say "toma vaso" because that doesn't make sense. You can say "toma un vaso" not because you want the other person to have just one, but to be gentle, and because you normally say it that way.

eran como a las cuatro Sal de aquí como las cinco CORRECCION: depende! por ejemplo: "eran COMO LAS cuatro" lo usas si solo quieres hacer referencia al tiempo, en este caso solo estas señalando la hora. mientras que: "(el evento)...era COMO A LAS cuatro" o "salí de aquí COMO A LAS cinco" lo usas cuando hablas de un evento(salí) que aproximadamente sucedió a esa hora COMO: en estas frases, es parecido a decir "aproximadamente" ******** toma asiento : das la orden de sentarse. toma un asiento: es porque hay varios asientos y puedes escoger. soy ingeniero: te defines como ingeniero. soy un ingeniero: eres un ingeniero entre muchos otros ingenieros.