There are two kinds of dageshim:  more here or this video
| 
 dagesh lene    video 
 | 
 dagesh forte    video 
 | 
| A dagesh lene makes the pronunciation of a BGDKFT letter hard; e.g. a ב is pronounced “v”. A בּ is pronounced “b”. | A dagesh forte doubles the pronunciation of a letter. | 
| Only occurs in BGDKFT letters. | Can be in any letter except a guttural. | 
| Never preceded by a full vowel. | Always preceded by a full vowel. | 
| Does not affect a syllable being open or closed. | A letter with a dagesh forte in it closes the syllable. | 
Note #2: the dagesh in a vav-conversive will never appear in the first letter of the verb root.