It is common for less to be used when fewer would be appropriate.* The correct usage basically boils down to whether you’re referring to something that can or cannot be counted.

Use fewer with countables and less with uncountables.

If you mean smaller in number, use fewer. This applies when you are referring to people or things in the plural. You can count them, e.g. books, students.

If you mean smaller in quantity, use less. This applies when you are referring to something that doesn’t (normally) have a plural. You cannot count it, e.g. music, mud.

You should also use less when referring to a number on its own and expressions of measurement and time. Less is also correct when referring to age and sums of money. This is because you are referring to the total amount rather than individual units. (This is usually expressed as less than. For example, less than sixty miles, less than £80, less than four years.)

* The use of less in front of nouns that can be counted (count nouns) is not a new phenomenon. King Alfred the Great did it in c.888. The rule against using less in front of plural count nouns is usually traced back to one grammarian writing in 1770 (and it was not intended to be as strict as it has become in modern usage). It will be interesting to see if using less with count nouns becomes acceptable once more.

Defuse

• to remove the fuse from (an explosive device)
• to reduce the tension or danger in (a difficult situation).

Diffuse:

• to spread over a wide area (or between a large number of people)
• to mix (a gas or liquid) through or into a surrounding substance
• to cause light to spread evenly
• spread out over a wide area; not concentrated or localised
• lacking conciseness or clarity.

In my experience, it is common for people to get confused when using ‘tact’ and ‘tack’.

Tact:

• a sense of (and an ability or will to use) the best and most considerate way to deal with people (so as not to upset them)
• skill in handling difficult situations or issues

Tack:

• a course of action or policy, or a way of dealing with a problem or situation
• a small, short sharp-pointed nail with a large flat head; to fasten something or fix in place with a tack or tacks
• to add something to an already existing whole – ‘to tack something on’
• a drawing pin (in North American English)
• a long loose temporary stitch used to fasten fabrics together; to sew something with long loose temporary stitches
• the riding harness for horses, including saddles and bridles
• the act of changing course by turning a boat’s head into and through the wind (creating a zigzag course to make progress against the wind); the boat’s course relative to the wind’s direction; the distance sailed between tacks
• a rope for securing a certain type of sail; the corner a rope is fastened to
• being sticky (as a quality or property)

Task:

• a specific piece of work to be done or undertaken
• an unpleasant or difficult job or duty
• to assign or give a job or piece of work to
• to make demands on someone’s resources or abilities
• to criticize or rebuke – ‘take to task’

Thank you to Johanna Levene aka Afthead for suggesting this post.