When they occur immediately before of after the dentals /θ,ð/, otherwise alveolar /t,d,n,l/ are realized as dental consonants. Before GA /θ,ð/, the tip of the tongue forms a complete or partial closure with the inner edge of the upper teeth rather than with the alveolar ridge, to facilitate the pronunciation of the following dental fricative or approximant. Here are some examples of dental /t,d,n,l/ before /θ,ð/:
right there [raɪt̪ʔ ðɛr]
When /t,d,n,l/ follow GA /θ,ð/, the dental contact is prolonged but changes from a narrowing for /θ/ or a light contact for /ð/ into a complete closure for [t̪,d̪,n̪] or a partial closure for [l̪]; at the same time the blade of the tongue may be raised to touch the alveolar ridge. Here are some examples:
both times [ˈb̥oʊθ ˈt̪ʰaɪmz̥]
breathe deep [b̥riːð d̪iːp͜ʔ]
with luck [wɪð ˈl̪ʌk͜ʔ]
Concentrate on the dental articulation of /t,d,n,l/ before /θ,ð/, as in eighth, wealthy, health, enthusiasm, at the time, good things etc. In a rapid pronunciation of /d,n,l/+/ð/ sequences you can actually omit /ð/, especially in unaccented syllables, provided you pronounce a dental /t,d/ or /l/. Thus, in there, said that, all those may be pronounced [ɪn̪ɛr, sɛd̪ət, ɑːl̪oʊz], in anything but the most careful styles. The pronunciation of dental /t,d,n,l/ after /θ,ð/ is not usually particularly difficult, once /θ/ and /ð/ are pronounced correctly.