Questions & Answers about No relaciones una cosa con la otra.
Why does the verb relacionar end in -es here (relaciones)?
Because this is a negative command for tú ("don't connect"). To tell someone not to do something, Spanish uses the present subjunctive form. For -ar verbs like relacionar, the negative tú ending is -es. If you were telling them to connect it (a positive command), you would just use relaciona.
Why is the preposition con used instead of a?
Why do we need the article la in la otra?
La otra translates to "the other [one]". The feminine article la is required here to match the gender of the noun we are talking about, which is cosa. If you dropped the la and just said con otra, it would change the meaning to "Don't connect one thing with another (different) thing" rather than "with the other thing".
How could I use this sentence's structure with 'lo de' to say 'Don't connect yesterday's situation with today's'?