There are time when reading in Welsh you'll hit on words you don't know.  Sometimes you'll be able to pick up the sense even without help, sometimes you'll have to resort to a dictionary -- somewhere between the two fall the skills of picking out the meaning from context and using root and compound words.

This can be a good way to figure out the meaning without breaking your flow to go for a dictionary and the mental effort put in also means you're more like to recognise the word when you see it again.

An example...

Sadwrn yw planed ail fwyaf Cysawd yr Haul. Sadwrn yw'r chweched blaned oddi wrth yr Haul.

Enwyd y blaned ar ôl Sadwrn, duw ym mytholeg Rhufeinig. Gwybyddir am Sadwrn ers amserau cynhanesyddol. Galileo oedd y cyntaf i edrych arni gyda thelesgop ym 1610.  (http://cy.wikipedia.org/wiki/Sadwrn_(planed))

The words in bold are the ones we'll 'figure out'.

Cysawd - appears to be one of those words which you know or don't.  But the words around it give the game away entirely.  We're talking about Saturn, planets, the sun and the sentence says it's the second biggest in the Sun-something.  It can only be the Welsh for 'Solar System'.  (cysawd on its own is just 'system')

The next two, enwyd and gwybyddir, are examples of the common occurence in Welsh where the 'ending' and, in the second case, a vowel have changed to alter to the tense.  In 'enwyd' the key bit is enw - name, and 'gwybyddir' has come from gwybod.  Both are examples of a passive sort of past tense.  "Saturn was named" "Saturn has been known..."

"Undoing" the ending and letter changes in order to figure out the original word is something you're likely to have to practice even when using a dictionary as many of the 'with endings' versions of the words won't appear in that form!

The next example is a compound words made up of more than one element.

Cynhanesyddol - 'hanes' means 'history' and 'cyn' means 'before'.  So the first part of the word - 'cynhanes' - would mean 'prehistory'.  The '-ol' on the end is a suffix that often turns a noun into an adjective, which makes the whole word mean 'prehistoric'.  "Saturn has been known since prehistoric times."

Cyn is a useful one often used in a similar way to the English prefix 'pre'.  (My favourite example being 'cynllunio' meaning plan - or literally "pre-picture"!)

There's a number of these which are useful to know.

cyn- (similar to 'pre')

an- and di-  (indicating a negative similar to English un-, dis- or in-)

is- (similar to sub-)

A brilliant book for this sort of thing is the massive "Welsh Roots and Branches" by Gareth Jones.