As explained under Rule Three of my Basic Rules post, consonants usually should not stand next to each other. If a word has two consonant sounds next to each other with no vowel between them, the consonants combine to create a new letter called a conjunct character. In some conjunct characters it’s easy to see the characters that created them, as elements of each character might be included in the design of the letter. In other cases, conjuncts might appear completely different than the characters that created them so it’s harder to guess the sounds they make. Unfortunately in these cases the best you can do is try to memorize them and learn them as you see and hear them in words. A few conjunct characters have different pronunciations from what you might expect, but these are not as common. 

Below is a list of commonly used conjuncts accompanied by the characters that combined to make them, along with romanizations that include their resultant pronunciations. The starred (*) conjuncts are pronounced differently than expected. Bear in mind, this is not an all inclusive list as there are many conjuncts in existence.

ক্ত (ক + ত) | kt (k + t)

ঙ্গ (ঙ + গ) | ṅg (ṅ + g)

জ্জ (জ + জ) | jj (j +j)

ঞ্চ (ঞ + চ) | ñc (ñ + c)

ট্ট (ট + ট) | ṭṭ (ṭ + ṭ) 

ণ্ঠ (ণ + ঠ) |  ṇṭh (ṇ + ṭ)

ত্ত (ত + ত) | tt (t + t)

দ্ধ (দ + ধ) | ddh (d + dh)

ন্থ (ন + থ) | nth (n + th)

প্ত (প + ত) | pt (p + t)

ম্ম (ম + ম) | mm (m + m)

র্ক (র + ক) | rk (r + k)

ষ্ট (ষ + ট) | ṣṭ (ṣ + ṭ)

স্ত (স + ত) | st (s + t)

ক্র (ক + র) | kr (k + r)

ঙ্ক (ঙ + ক) | ṅk (ṅ + k)

চ্ছ (চ + ছ) | cch (c + ch)

ঞ্জ (ঞ + জ) | ñj (ñ + j)

ণ্ট (ণ + ট) | ṇṭ (ṇ + ṭ)

ণ্ড (ণ + ঢ) | ṇḍh (ṇ + ḍh)

ত্র (ত + র) | tr (t + r)

ন্ত (ন + ত) | nt (n + t)

ন্ধ (ন + ধ) | ndh (n + dh)

ব্দ (ব + দ) | bd (b +d)

ম্ভ (ম + ভ) | mbh (m + bh)

ল্প (ল + প) | lp (l + p)

ষ্ণ (ষ + ণ) | ṣṇ (ṣ + ṇ)

স্থ (স + থ) | sth (s + th)

Some oddballs:

*ক্ষ (ক + ষ) | kh/kkh between vowels (k + ṣ)

*হ্ন (হ + ন) | nh (h + n)

*জ্ঞ (জ + ঞ) | g/gg between vowels (j + ñ)

*হ্য (হ + য) | jjh (h + y)

Many conjuncts are not included in this list. If you come across a conjunct that isn’t here, you can check this chart. It may take a little to skim, but so far it seems like everything is on there.