A lot of people who just started to learn German don’t use contractions such as im, am, um, and zum, which can lead to a couple of funny situations, since there is a very subtle difference in using it vs not using it.

Im is a contraction of in and the dative article dem. If you speak about something general, you have to use the contraction im. If you are talking about a specific object, you can use in and the dative article dem to stress that you are talking about this specific object.

There are however, certain rules, which specify cases where you cannot substitute im with in dem so let’s look at those cases a little bit closer to fully understand this topic.

Nouns Which Are Actually Adjectives

You can easily make nouns out of almost every German adjective and some of them can be used together with im. Whenever this happens, you are not able to substitute im by in dem.

For example:

Im Glück
Im Allgemeinen
Im Großen und Ganzen
Im Prinzip

Whenever you feel like the noun you are using is originally an adjective and only derived from, it is safer to use im instead of in dem. You can recognize some of the nouns that were originally adjectives by their endning.

Those nouns oftentimes end with -heit or -keit such as:

Die Faulheit – faul – lazy
Die Schläfrigkeit – schläfrig – sleepy
Die Schlauheit – schlau – smart
Die Schönheit – schön – beautiful

General Locations

If you are talking about general locations such as the sea, the garden or the forest, you will also have to use im.

Im Meer – In the sea
Im Garten – In the garden
Im Wald – in the forest
Im Haus – in the house

Time

When talking about the time in general, you have to use im instead of in dem. This applys to seasons, except for when you talk about a season in a specific year.

Im Frühling – in spring
Im Sommer – in summer
Im Herbst – in autumn
Im Winter – in winter

If you want to talk about a specific season in a specific year, you can use dem, but I would advise you to use diesem instead.

In this winter we did not have a heater.
In diesem Winter hatten wir keine Heizung.

Skills

If you talk about having a skill, by using the phrasing with im, you are not able to separate im. This applies for sentences that use “to be good at” in English.

I m good at painting.
Ich bin gut im Malen.

I m good at running.
Ich bin gut im Rennen

I m good at cooking.
Ich bin gut im Kochen

Using A Noun Instead Of An Adverb

In German, you are able to use nouns instead of certain adverbs. You will always have to use im in this case. This applies to adverbs of manner/way

I told him jokingly…
…sagte ich zu ihm im Scherz

English speakers can have troubles with this since you it is not always possible to translate im into in the. For example:

Im Vergleich – in comparison
Im Ernst – in earnest
Im Allgemeinen – in general

Conclusion

It is generally recommended to use the contractions if you are talking about something in general. Most of the times, if you don’t want to specifically stress some word, you should just keep using im, even if you are talking about something specific.

If you do this, you will not make a mistake, in about 99% of all the times that you use it. I myself oftentimes don’t use in dem, and I would only use it after someone spefically asked me to specify the object.