Earlier I thought I found the rule în was followed by noun with definite article but the last examples have been noun plus indefinite article. So how do you work it out please.
Unless the noun is modified (e.g. by an adjective), it has no article when following "în", if referring to a specific thing. But, for example, if the keys are in a purse, it does take the indefinite article: "Cheile mele sunt într-o poșetă." ("într-o" is a mandatory contraction of "în o").
Thanks, that's interesting. So far we've not practised the mandatory contractions only had to chose them so I think that's helpful too.