Ambiguity
A sentence is grammatically ambiguous when it could have more than one meaning. For example:
- Fuel cells were investigated for space flights because they were developing fast.
- NASA looked at a battery and a solar panel but ruled it out because of its weight.
Which was developing fast, fuel cells or space flights?
What was too heavy, the battery or the solar panel?
Grammatical ambiguities creep in when you have two items followed by a pronoun that could be standing in for either item/person. (A pronoun is a word standing in place of the person or item's name, e.g. I, him, they.)
- fuel cells, space flights: pronoun = they
- battery, solar panel: pronoun = it
To avoid grammatical ambiguity, the example sentences should read:
- Fuel cells, which were developing fast, were investigated for space flights.
(The sub-clause is now next to its subject, so the meaning is clear.)
- NASA looked at a battery and a solar panel but ruled out the battery because of its weight.
(The repetition of 'battery' make the meaning clear.)