Tool or means
What do we use to do it?
Meaning
A tool or means is what someone uses to carry out an action. It can be an object, a body part, a way of traveling, or a way of sending information. Languages express this with separate words, endings, or context; some have no special marker for it. Tool or means identifies what is used, while How the tool is marked looks at the words or endings that connect it to the action.
How to spot it
Ask what is used to carry out the action, rather than who does it or who benefits from it.
In your native language
English commonly uses with to name a physical tool. It often uses by for a way of traveling or sending information, as in by train or by email. The choice depends on how the tool or means is described, so there is no single preposition for every case.
Mia opened the door with a key.
With names the physical tool used to open the door.
Please send the photos by email.
By email names the way the photos are sent.
We got there by train.
By train describes the way of traveling; train has no article here.
In your learning language
Spanish usually expresses a tool with con, a form of transportation with en, and a communication channel with por. The choice depends on the kind of means being used, rather than on one general tool ending.
Corté el pan con un cuchillo.
I cut the bread with a knife.
Mañana vamos en tren.
We're going by train tomorrow.
Te mando la dirección por correo electrónico.
I'll send you the address by email.
Learners whose language uses one form for tools and transportation may extend con to vehicles. Communication methods also need their usual Spanish pattern rather than a direct replacement for a general means marker.