It never fails.
You’re in middle of a conversation and you lose your train of thought. Maybe you got distracted, or maybe you’re like me and the mind just isn’t as sharp as it used to be.
Regardless of the reason, you’re going to have to explain why you have you that confused look on your face.
Let’s get to it.
Telling someone you forgot what you were going to say is easy. Let’s take a look at a few of the options we have using standard Spanish.
Olvidé lo que iba a decir
I forgot what I was going to say
No recuerdo lo qué iba a decir
I don’t remember what I was going to say
Te iba a decir algo, y me olvidé
I was going to tell you something, and I forgot
Olvidé lo que te iba a decir
I forgot what I was going to say
Se me olvidó lo que te iba a decir
I forgot what I was I going to tell you
I want to point out a couple of things about the examples, so lets take a closer look at these sentences, starting with lo que.
Lo que is a fixed expression, meaning “what”. Mariela in Spanish has one of the best explanations I’ve seen so far.
“Lo que” and how to use it in Spanish
You also probably noticed I used three different ways to say “I forgot”:
me olvidé, olvidé and se me olvidó.
If you’re wondering what the difference is between them, don’t. From a conversational and practical perspective they all mean the same thing. Grammatically they are different, but that’s complicated and we aren’t going to discuss that here.
Now it’s time to explore some colloquial/informal options that will make you sound more native. Besides, if you talk to enough people or watch enough Spanish TV you’re bound to hear them, so it’s best to be prepared.
Ya se me fue lo que te iba a decir
I forgot what I was going to say
Using the construction se me fue is very colloquial, but also common. Se me fue literally means “it left me”, and the entire phrase would be something like:
What I was I going to tell you left me
That’s not very good English, which is why is best to just think of this as “I forgot”. Spanish speakers use the construction se me fue in a lot in of other contexts as well, so you’ll eventually see this elsewhere.
Let’s look at some idiomatic options. I’m going to translate them literally, but remember figuratively they all mean “I forgot what I was going to say” or “I lost my train of thought”.
Here’s a very Mexican way:
Se me fue el avión
The airplane left me
And this one you’ll hear in Colombia:
Se me fue la paloma
The dove left me
This one is used in the Dominican Republic and likely throughout the Caribbean:
Se me fue la guagua
The bus left me
By the way, in the Dominican Republic, Puerto Rico and Cuba guagua means bus. It has a different meaning in other countries, or may not even exist at all.
And lastly, this one is common in Spain.
Se me fue la pinza/olla
And that’s it for today. Now you have a variety of ways to be able express yourself like a native speaker.
And if you have a hard time remembering these phrases, then 
you can also get them on your Android phone with the My Spanish Phrasebook app.
Developed by me, it has 1800+ useful phrases for expats, travelers and Spanish learners for everyday situations. It can be hard to remember all that Spanish floating around in your head, so this is the perfect tool to help you remember those Spanish phrases when you need them the most.
¡Espero que te sirva!