This week’s error involves the incorrect adding of a preposition. In Czech, you date ‘with’ someone, but in English we don’t include the preposition, we simply date them.
There is no fixed rule about how long you can say you are ‘dating’ someone before describing the relationship as something else. You can say you’ve been dating someone for ten days, ten months, or ten years. However, too long perhaps sounds a bit strange. Imagine two old people who have been together for 40 years without ever getting married. „I’ve been dating Jaroslava for 40 years!“ Maybe it’s just romantic!
Since we’re talking about relationships, another common mistake is when people say, „They had a sex.“ In English, sex is an uncountable noun, and you can’t use it with the article ‚a‘. So, instead, you simply say, „They had sex.“
Here’s a correct sentence with both of this week’s mistakes:
My friend has been dating Honza for three years, but last week she had sex with his brother! What a terrible woman!