The following collection of grammatical errors are fine examples of a number of complex and indeed very complex mistakes in English grammar. The corrections are based on Swan’s Practical English Usage (2005).
– PREPARATORY IT
“In this theory is explained how it is possible that these languages are still spoken and very different from each other.” = In this theory it is explained how it is possible that these languages are still spoken and very different from each other.
(446) & (178.2) Wrong ellipsis. Preparatory it as (provisional) subject; it at the beginning of the clause, the (finite) clause as subject at the end.
– PROVISIONAL THERE
“At first the Vikings and the Anglo-Saxons lived together in a fairly friendly way so there were borrowed a lot of words from Scandinavian.” = At first the Vikings and the Anglo-Saxons lived together in a fairly friendly way so a lot of words were borrowed from Scandinavian.
(587) Provisional or introductory there is only used as subject to say something exists somewhere; it is mainly used with lexical be, but never in the passive.
– CONDITIONAL IF
“If we play tennis, I am sure I would win.” = If we play tennis, I am sure I will win.
(256-263) First conditional – to describe an event likely to happen now or in the future: if + present tense, will + infinitive.
“Many problems that married couples face could end if they would reinstate a ‘dating attitude’ in their relationships.” = Many problems that married couples face could end if they reinstated a ‘dating attitude’ in their relationships.
(256-263) Second conditional – to describe non reality in the present or future: if + past tense, could/would + infinitive.
“I realise that if I would have had these children under my wings for a longer period of time, I would have known who is fast and who is slow.” = I realise that if I had had these children under my wings for a longer period of time, I would have known who is fast and who is slow.
(256-263) Third conditional – to describe a past situation that did not happen: if + past perfect tense, would have + past participle; no would in the main clause.
See other: Notes On English Grammar