Using appropriate terms and correct search logic is essential to constructing searches that will find useful articles.

You can combine MeSH and  keywords in PubMed searches. Keywords are particularly useful for finding recent articles.

You can maximise your retrieval of relevant citations by using logic statements such as 

chewing gum AND tooth decay

PubMed is programmed to find significant words, and to assume that you want each word in your search to be included in every reference found UNLESS YOU TELL IT OTHERWISE. Using logic statements is the best way to ensure that PubMed finds what you're looking for when using keywords.

Open PubMed.
Type in the search chewing gum AND tooth decay and click on Go (or press enter).

Click on Details to see how PubMed has searched.

PubMed shows how it has enhanced the search and combined your search terms.

MeSH
PubMed will try to find a MeSH that corresponds to each of your search terms. Notice how the MeSH dental caries has been used to find records for articles on the search term tooth decay.

All Fields
PubMed searches for terms designated as All Fields in just about every part of the citation including title, abstract, first author's address and many other areas. Because PubMed pulls phrases apart and searches individual words, this can often find citations that aren't relevant. See the section below How To Avoid Finding Too Many Irrelevant Citations to reduce this problem

Logical Operators
Notice that the words in between the search terms (OR, AND, NOT) are in capital letters. These are called logical operators and tell PubMed how to combine the terms entered by the searcher. You must always use capitals for logical operators in PubMed.

Explanation of AND logic
AND logic is used to find citations for articles that include both the terms on either side of the AND.

hamburger AND e coli
Finds only those articles that are about both the bacterium E. coli  and hamburger. In the graphic above, the green area common to both circles represents the articles with both terms.
 

Explanation of OR logic
OR logic finds citations of journal articles that have any one or more of the terms linked by OR.
A search of teeth OR tooth OR dentition finds citations for all the articles that have any one of these terms, any two of these terms and all three of these terms.

Asking Answerable Questions
One of the skills of database searching is to be able to frame answerable questions. In the clinical sciences it's often useful to frame your questions to include
the group of people of interest
(e.g. children using electric or sonic toothbrushes),

what you might do for them
(e.g. decide on the incidence of plaque),

what you might use for comparison
(children using standard toothbrushes)

and what outcome you might be looking for
(e.g. better oral hygiene)

So it helps to remember PICO (Population, Intervention, Comparison, Outcome)

Your question might be
Do children using electric and sonic toothbrushes have a lower incidence of plaque than children using conventional toothbrushes?

To help you search you can construct a grid with the main concepts at the head of a series of columns. You don't always need to include all the aspects mentioned.
  

Next think of synonyms, alternative terms, singualr/plurals etc. for each of these terms.
List these new terms underneath your original terms.   
Plurals, and British/North American spelling variations are worth including in your lists.

How To Avoid Finding Too Many Irrelevant Citations
Typing topic terms means that PubMed will search in All Fields.
This isn't always the most efficient search method because PubMed may find many articles that aren't about your topic.

Instead
first check the MeSH database to find at least one MeSH for each concept you are searching.
Always try to include at least one MeSH in each column if possible.
This ensures that you include in your search all the citations indexed by that subject heading.

Add [mh] to your term in your grid to make PubMed search it in the MeSH field
e.g. child[mh]

Next search for the same term in the titles and abstracts of citations.
Do this by searching
child[tiab]

Add to this other search terms that might be used by authors in their titles and abstracts to describe the content of their articles. Including other non MeSH terms for the concept will help you to find citations to articles that don't have a MeSH. Searching in the titles and abstracts of citations is more specific than allowing PubMed to search in All Fileds.

Truncation
PubMed uses the asterisk *, as its truncation symbol. Adding * to a word stem will get PubMed to search for keywords beginning with the letters that start with the letters to the left of the *.

prosth*[tiab] will search for prosthetic[tiab] OR prosthetics[tiab] OR prosthodontic[tiab] OR prosthodontics[tiab] etc

Never use truncation with a MeSH. It interfers with automatic explosion.

Your logic grid will now look something like this

Entering Searches into PubMed
When you type your search in the PubMed query box, the terms in each of your columns must be joined by OR logic and enclosed in round brackets.
The terms from different columns are joined by AND logic.
Enclosing the terms from each column in brackets ensures that they are combined using OR logic before the AND logic is used by the PubMed program.

The search above becomes

(child[mh] OR child*[tiab] OR boy*[tiab] OR girl*[tiab]) AND (electric toothbrush*[tiab] OR sonic toothbrush*[tiab] OR power toothbrush*[tiab]) AND (conventional toothbrush*[tiab] OR manual toothbrush*[tiab]) AND (dental plaque[mh] OR plaque[tiab])

This can be typed in the PubMed query box.

Here are some of the results.

Now answer Question 4.

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