In English, the infinitive marker is to (which must not be mixed up with the preposition to), and in Swedish the infinitive marker is att (which must not be mixed up with the subordinating conjunction att).

The infinitive form of the verb looks just like the base form of the verb, i.e. the first verb that is mentioned when you give the principal parts of a verb (go - went - gone; gå - gick - gått).

(i) To be nice to everybody is not easy when you just want to sleep.

(ii) Han är van att laga sin mat själv, men föredrar att få den serverad.

Many of us have been taught (both in English and in Swedish) not to split our infinitives. To split an infinitive is to put something (normally an adverbial of some sort) between the infinitive marker and the infinitive, as in the following examples, where the infinitive marker and the infinitive appear in boldface:

(iii) To really tell you the truth, I wanted to finally be successful.

(iv) Att återigen slå världsettan i Paris är en bragd att genast skriva hem om.

We can see that there is an adverb phrase (really) functioning as adverbial between the infinitive marker to and the infinitive tell in (iii). Similarly, there is a relatively short adverbial between to and be (iii), between att and slå (iv), and between att and skriva (iv). In other words, all the infinitives in examples (iii) and (iv) are split.

In the old days, splitting an infinitive was considered to be something really bad from a grammatical point of view, mainly because it was impossible to split infinitives in Latin. Nowadays, however, people have realised that there are cases when splitting the infinitive is quite OK, and sometimes even more natural than keeping the infinitive marker and the infinitive right next to each other.

If we look again at examples (iii) and (iv), it is hard to say that the sentences would be considerably better if the infinitives were not split. The adverbial really, for instance, seems to have its natural position in this sentence just where it is now.

In other cases, we split our infinitives to create certain rhetorical effects. One famous example is the Star Trek quotation in (v):

(v) To boldly go where no man has gone before

However,even though there is no general ban on splitting infinitives in Swedish or English nowadays, we ought to be aware of the following facts, principles, and rules:

(1) In Swedish we can put heavy (long) elements between the infinitive marker and the infinitive. This usage should be avoided in English. When we split infinitives in English, what we put between the infinitive marker and the infinitive is almost always an adverbial in the form of a one-word adverb phrase (see the examples above) or two conjoined adverbials in the form of one-word adverb phrases (as in vi).

(vi) I would like you to kindly but firmly ask him to leave the premises.

(2) Do not use the negative adverbs not and never to split your infinitives in English. In Swedish, however, this is generally perfectly fine, as in (vii):

(vii) Att aldrig få som man vill är lika illa som att inte få betämma någonting.

(3) Do not split your infinitives for no particular reason at all. If there is an alternative way of expressing oneself that naturally conveys the same information without losing the intended rhetorical effect, please do not split the infinitive.